My bare-bones app has suddenly started working, FYI. Seems like it still required a change to the Game Center config (like a fake leaderboard, etc) to force the push to Game Center servers

It's an ongoing issue: https://developer.apple.com/forums/thread/698731 Apple Support is apparently looking at it.

I just got a response from Support as I'm having the same issue, and also with a bare-bones app like yours. They directed me to the documentation... sigh. I've pointed them toward this thread too.