Hi everyone, I would like to provide update on this thread as I was working together with Microsoft team and MVP Rory Braybrook to find the root cause of this issue.
First of all, here is the answer I got from the person from Microsoft:
Looking deeper this is a pretty generic error code - there are many ways you might end up miscoding the login domain such that you end up here. We have two fixes we'll be working on this quarter to help avoid this - fixing the issue Rory brought up where you get shown Entra ID configured samples in the App Registration pages and also updating the endpoints shown in the App Registration page which still point to login.onmicrosoft.com.
One more important thing is that samples were wrongly configured. You can read Rory's post here:
https://medium.com/the-new-control-plane/using-entra-external-id-ciam-with-the-msal-samples-86e6de6a8f20
One of the important things is the correct format for the authorize endpoint: https://<TENANT_ID>.ciamlogin.com/<TENANT_ID>/oauth2/v2.0/authorize
If you configure the sample with all above recommendations and you still face the issue it means that probably one of the issues mentioned by the person from Microsoft is happening in your preview tenant. The team is working to fix all potential issues this quarter.
I hope this will help you.