Just a guess: when the fight begins, you move the player to a room called the forest battle, right? But your “every turn” rule has set default monsters for the region in which the player was before you moved her to the forest battle room. Could this be the cause of the problem?
(By the way, if you are making a combat heavy game, you might want to check out my combat extension Inform ATTACK. Even if you do not want to use it, either the ideas or the code could serve as inspiration. If you’d like to see it in action, check out my game Kerkerkruip.)