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