I accepted a mailed arena challenge, and won it. But now the result of that battle is always shown in the arena: I've tried battling with several other pets, but can't see if they're winning or losing. All I see is that old result:

"You entered a battle with trinity and you won!"

Already tried logging out etc, but that didn't fix it. 

Thank you for reporting. The issue must be gone now. The very next battle will still show the old notice but all others must work correctly.