External (guest) users can't access my registered application

We have a FileMaker application registered with Entra ID, using OAuth, for internal and external (guests) users in my organization.

Since January 19th, external users have been encountering a different authentication process, which results in a 404 error (see images below).

No changes were made to the Entra ID or the application configurations before this change in behaviour.

It seems that logging in to a personal account results in an incorrect token for the redirect URL, which does not happen when logging in with organizational accounts.