Hi @JamesTran-MSFT , thanks for getting in touch.
I have solved the issue now. My problem was due the scope I was requesting when authenticating. For some reason, in the Program.cs file, when configuring the Msal Authentication provider. When I included the Default scope for my Server API, I managed to get the scope name wrong (the one that needs to be qualified with the AD organisation).
Once I corrected this, I was able to get the correct token returned and it's all working now.
Thanks.