I think the mushrooms might be spawning poison pool or poison cloud elemental spots underneath themselves every turn. Then if you lay the burn on it - they do that every turn.
I've actually used that as a battle tactic. I was outnumbered and outgunned but I lured them into a poison gas that respawns and fire arrows. Alternatively poison gas arrows explode on fire. Sort of makes me think magic tart + exploding arrows would an inexpensive sick combo. Or flare + poison gas arrow
