Is there some kind of code size limit for UWP user controls to work in Xaml islands? (exit code 0xc000027b)

I need to get xaml islands working with my project. I started with the small samples, works great. I modified our large UWP application to provide a UWP user control to WPF and when I run, it looks like it is starting but then just suddenly ends, no errors but exit code 0xc000027b.

Breaking this down I have reached the point that:

1. I create a new fresh small test project and small user control, it works.
2. I add all the references and libraries etc. of the main project to the small test project, it still works
3. I add all the source of the main project to the new test project, but without any of it being instantiated, and still only showing the small test user control. So all the code it there but it isn't doing anything. Now when I start, the problem occurs. A breakpoint on me.initialise in App.xaml.vb does not get hit. It is a big solution with about 15 projects.

So it seems that 0xc000027b is resulting from having a large volume of code associated with the project, even if it isn't being used. Seems unlikely I know. Any suggestions about how to solve this or diagnose it deeper? 0xc000027b seems to have been around for a while with a number of odd causes.

This appears in the event log (AviHost is my WPF app):

Faulting application name: AviHost.exe, version: 1.0.0.0, time stamp: 0x5d7bb03a
Faulting module name: Windows.UI.Xaml.dll, version: 10.0.18362.418, time stamp: 0x253810c2
Exception code: 0xc000027b
Fault offset: 0x0000000000712dc0
Faulting process ID: 0x6868
Faulting application start time: 0x01d594bda617d418
Faulting application path: C:\Users\ianb\Source2\Workspaces\AviUwp\AviHost\bin\x64\Debug\netcoreapp3.0\AviHost.exe
Faulting module path: C:\Windows\System32\Windows.UI.Xaml.dll
Report ID: 0ab8cf51-5e72-4d12-82d5-b9b6b6e2bf78
Faulting package full name: 
Faulting package-relative application ID: