Ok this was a tricky one. These bats were once doppelganger ( with fire immune ) then change to vampires ( with fire weakness ) and then changed to bat. None of this fire status were reseted on each monster type change resulting in a very rare case of immune & weak to fire bat. It will be corrected on next version ( probably the final one :D )
Nice detailed report, I could find the bug in no time thanks to it !