Bad username/password error when initiating a process cube job from SQL Agent using a proxy credential

When initiating a process cube job from SQL Agent using a proxy credential I get a bad username/password error, these details are correct.

If I run the process cube task as the SQL Agent service account, and if I have the SQL Agent service account as an admin on the SSAS instance then this works fine, however the spec I am building out infrastructure to requires me to use a proxy credential to do this.

I have created a credential in SQL Server security. I have created a proxy in the AnalysisServicesCommand subsystem with the above credential. The credential has access to the database the Analysis Services needs to build the cube, the cube buildd successfully running manually from SSAS interface The password is definitely correct as I can run Notepad as the user, I have even tried with my own sysadmin user as the proxy credential and got the same error. This is what I get from Windows Event Viewer.

Failure Information:
    Failure Reason:     Unknown user name or bad password.
    Status:         0xC000006D
    Sub Status:     0xC000006A