You are not alone! I have had this problem for the last two days. I tried absolutely everything, what I know and founded in the Internet and nothing helped. Did you try to send your app to internal TestFlight review? I'm waiting for review completion and hope it may fix this problem.

I found this post https://developer.apple.com/forums/thread/698731?answerId=702089022#702089022 I read the solution and one more time went to App Store connect, turned off and then turned on GC with leaderboards and achieves and it worked for me! And GC is finally work in old builds, which I made on friday. I don't know what it was. May be you must wait for days after you turn on GC for the first time, because GC processing is so long. Or it may be some Apple bug.