Thank you for your detailed post!
Can you try using the Redirect URI -
https://oauth.pstmn.io/v1/callback within your Azure AD App registration, to see if that helps resolve your issue.
I was able to successfully authenticate within my Postman environment using that Redirect URI:
If you have any other questions, please let me know.
Thank you for your time and patience throughout this issue.
Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.