I was fighting a lone Forest Sprite in the Lost Grove. It was almost dead, had an HP regen buff on itself, and right when its HP would have run out, the sprite simply disappears when I hit the end turn button, and a message comes up that my pet doesn't have a valid target. I used skip turn for all my pets, and the sprite reappeared. I was able to finish the battle then. But that was weird. I'm in stage 3 of the Lost Grove, and it was near the barracks, if that matters.