In the challenge mode

That's not a bug mate.  Winning/Elimination/Withdrawal rewards are all different.  If you check the tab  Withdrawal, you should see only coins against each round.  If you don't want to play, just let the match run it's course and if you get eliminated you still get the gems.  Unfortunately with withdrawal your rewards changed to the later.  EAs intention probably is to encourage people to play and not forfeit but that's the rules set for the event, tough luck.