@Shane Goodman
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.
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