Thank you! The bats turn into vampires when exposed to sunlight, and then die if the beam is on them while a turn is passed (while the vampire count dies immediately upon exposure). Do you mean that the bat didn't immediately die upon passing a turn?
Thinking about it, I tried to make it easy to discover that particular mechanic in the game (as one shouldn't have to read the description to understand the mechanics) and do so without text, but I didn't think it could be mistaken as a bug actually...