Nice! I see that many devs report this same problem and they confirm it started last Monday, Aug 18. Latest update from an Apple representative: Aug 21: Aug 22:
Aug 23: Apparently no success because at the moment I get "No leaderboards" and even "No achievements". Another reset.
I think that Apple processed the data chronologically and updated leaderboards with the latest score submitted, not with all, including the highest, score submitted. Makes kind of sense, it is just that I have been playing Meerkatz Challenge that when replaying a level doesn't submit the known high-score for that level but the achieved score of the replay. Admittedly more a bug in the game than in GC, but if we hadn't suffered the 5 days of GC outage, I wouldn't have lost those high scores ... ah well, living is complaining
The Game Center service is back working normal. If someone is still having problems, just log off and log in or close GameCenter from the background should work.