There is a problem with the bat spell where it wont fly up as high like it use too, also sometimes it wont respond to using space bar to fly higher, so you carn’t predict where to land safely.
Update: I read that this might be a known issue, and i also read that casting other spells when using call of nergal / bat flying causes the desync however you carn’t cast spells while flying so it must be another reason.