visual studio project always rebuilding but pdb not updated

I've turned on verbose logging.

I'm always getting this:

1>FastUpToDate: Input 'C:\Development\Source\DotNet\common\Bentley.APM.Common.ODBC\obj\project.assets.json' is newer (5/5/2020 8:52:58 AM) than earliest output 'C:\Development\Source\DotNet\bin\x64\Release\Bentley.APM.Common.ODBC.pdb' (5/1/2020 11:17:23 PM), not up to date. (Bentley.APM.Common.ODBC)

In the bin folder the dlls timestamp gets updated every time I build. But the corresponding pdb timestamp is from DAYS ago.

I think the pdb is only getting updated if I go and manually edit one of the files inside the project that keeps "rebuilding".

Here is some more output that looks somewhat concerning:

Failed to resolve all items referenced by 'Service Providers\Ivara.ServiceProvider.Interoperability\Ivara.ServiceProvider.Interoperability.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Failed to resolve all items referenced by 'common\Authentication\Bentley.APM.Authentication.Oidc.Client\Bentley.APM.Authentication.Oidc.Client.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Failed to resolve all items referenced by 'common\Authentication\Bentley.APM.Authentication.SSPI\Bentley.APM.Authentication.SSPI.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Failed to resolve all items referenced by 'common\Authentication\Bentley.APM.Authentication.Interfaces\Bentley.APM.Authentication.Interfaces.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Failed to resolve all items referenced by 'common\Authentication\Bentley.APM.Authentication.Common\Bentley.APM.Authentication.Common.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Failed to resolve all items referenced by 'common\IvaraCommon\IvaraCommon.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Failed to resolve all items referenced by 'Service Providers\Bentley.APM.ServiceProvider.GraphQL\Bentley.APM.ServiceProvider.GraphQL.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Failed to resolve all items referenced by 'Service Providers\Rest.Common\Rest.Common.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Failed to resolve all items referenced by 'Service Providers\Ivara.ServiceProvider.Interoperability\Test\Ivara.ServiceProvider.Interoperability.Test.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Failed to resolve all items referenced by 'Service Providers\Bentley.APM.ServiceProvider.GraphQL\Tests\Bentley.APM.ServiceProvider.GraphQL.Tests.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Failed to resolve all items referenced by 'Service Providers\Bentley.APM.ServiceProvider.GraphQL\UnitTests\Bentley.APM.ServiceProvider.GraphQL.UnitTests.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Failed to resolve all items referenced by 'Service Providers\Rest.Common.Tests\Rest.Common.Tests.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Failed to resolve all items referenced by 'Service Providers\Ivara.ServiceProvider.MobileRestService\UnitTests\Ivara.ServiceProvider.MobileRestService.UnitTests.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Failed to resolve all items referenced by 'Service Providers\Ivara.ServiceProvider.MobileRestService\Test\Ivara.ServiceProvider.MobileRestService.Test.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Failed to resolve all items referenced by 'Utilities\ServiceProviderHost\ServiceProviderHost.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Failed to resolve all items referenced by 'TestingInfrastructure\TestConfigurationApp\TestSettings.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Failed to resolve all items referenced by 'TestingInfrastructure\TestFramework\TestFramework.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
System.Runtime.InteropServices.COMException (0x80004005): Error HRESULT E_FAIL has been returned from a call to a COM component.
   at VSLangProj80.Reference3.get_SourceProject()
   at NuGet.PackageManagement.VisualStudio.VsCoreProjectSystemReferenceReader.IsProjectReference(Reference3 reference, ILogger logger)
System.Runtime.InteropServices.COMException (0x80004005): Error HRESULT E_FAIL has been returned from a call to a COM component.
   at VSLangProj80.Reference3.get_SourceProject()
   at NuGet.PackageManagement.VisualStudio.VsCoreProjectSystemReferenceReader.IsProjectReference(Reference3 reference, ILogger logger)
Failed to resolve all items referenced by 'EXPLink\EXPLinkTestSuiteExtensions\EXPLinkTestSuiteExtensions.vcxproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Failed to resolve all items referenced by 'Service Providers\Ivara.ServiceProvider.MobileRestService\Ivara.ServiceProvider.MobileRestService.csproj'. This message can typically be ignored. The issue may be resolved by fully restoring and building the solution. If the unresolved item is a project reference this can lead to an incomplete NuGet restore result and missing package references. To ensure that restore is able to find all projects verify that all projects are referenced correctly and exist on disk.
Restoring NuGet packages...
To prevent NuGet from restoring packages during build, open the Visual Studio Options dialog, click on the NuGet Package Manager node and uncheck 'Allow NuGet to download missing packages during build.'
Running restore with 12 concurrent jobs.
Reading project file C:\Development\Source\DotNet\common\Bentley.APM.Common.Extensions\Bentley.APM.Common.Extensions.csproj.

Edit - I am back in this state again:

1>FastUpToDate: Input 'C:\Development\Source\DotNet\common\Bentley.APM.Common.Logging\obj\project.assets.json' is newer (5/13/2020 5:25:07 AM) than earliest output 'C:\Development\Source\DotNet\common\Bentley.APM.Common.Logging\obj\x64\Release\Bentley.APM.Common.Logging.pdb' (5/11/2020 2:28:57 PM), not up to date. (Bentley.APM.Common.Logging)
1>------ Build started: Project: Bentley.APM.Common.Logging, Configuration: Release x64 ------
1>Target GenerateTargetFrameworkMonikerAttribute:
1>  Skipping target "GenerateTargetFrameworkMonikerAttribute" because all output files are up-to-date with respect to the input files.
1>Target CoreGenerateAssemblyInfo:
1>  Skipping target "CoreGenerateAssemblyInfo" because all output files are up-to-date with respect to the input files.
1>Target CoreCompile:
1>  Skipping target "CoreCompile" because all output files are up-to-date with respect to the input files.
1>Target _CopyFilesMarkedCopyLocal:
1>  Touching "C:\Development\Source\DotNet\common\Bentley.APM.Common.Logging\obj\x64\Release\Bentley.APM.Common.Logging.csproj.CopyComplete".
1>Target ApmUpdateVersion:
1>  C:\Development\Source\DotNet\common\Bentley.APM.Common.Logging\obj\x64\Release\Bentley.APM.Common.Logging.dll : Version resource updated
1>

This will keep building again and again with:

1>FastUpToDate: Input 'C:\Development\Source\DotNet\common\Bentley.APM.Common.Logging\obj\project.assets.json' is newer (5/13/2020 5:25:07 AM) than earliest output 'C:\Development\Source\DotNet\common\Bentley.APM.Common.Logging\obj\x64\Release\Bentley.APM.Common.Logging.pdb' (5/11/2020 2:28:57 PM), not up to date. (Bentley.APM.Common.Logging)

The pdb is over two days old and doesn't seem to get updated.

I'm thinking that is because

1>Target CoreCompile: 1> Skipping target "CoreCompile" because all output files are up-to-date with respect to the input files.

I had a similar problem.
Try deleting the .pdb and then restart visual studio & rebuild. This worked for me