Well... you must have it on Erase Event. Instead after winning the battle turn the event's 'Local Switch X' on. Create a new page - requirements as Local Switch X = ON, make the page empty and as a parallel process with no graphic.
If my memory serves me right, this shall fix a problem that occurs with many things also, like chests.
EDIT: If this solves your problem add [RESOLVED] to the topic title. And I find local switches much easier than normal switches for things like these. As if the game is long enough you'd use up 400 switches of chests!