Posting as a new thread as an update to https://developer.apple.com/forums/thread/693310?page=1#697161022 got no traction. See there for details. TL;DR - working code suddenly starts getting a 500 response from the Apple Music back-end; new tokens etc. don't help. Previous resolution came from Apple, claiming a server fault was to blame.
Just me, or are others seeing this fault now recurring?