Trouble with GKTurnBasedMatch endTurnWithNextParticipants:turnTimeout:matchData:completionHandler:
I had the same problem (or at least similar), but found a way out of it. The object you recieve (gamedata) should not be passed around like a pie. In my project, I needed to take the data, convert them to game objects, process them with the data that was handeled, and then convert it back to a data package, and then send it to the next player.
To me, it is the only way you can safely process the data, without mutating the original source.
Best of luck, hope it helped.