Good catch, and your screenshot is the proof — one bat is floating fine and the one right next to it is half-buried in the floor.

Stirges are flying monsters, so instead of standing on ground they bob up and down around the spot the map file gives them. The catch is that the map file stores that spot AS the floor line (it's the sprite artwork itself that lifts a bat off the ground), so our bob was swinging about 20 pixels BELOW the cave floor on every downbeat. The safety check that was meant to stop that only ever looked for ground underneath the bat — and once the bat was already under the floor there was no floor left underneath it to bump against, so the check quietly did nothing at all. That's why some bats look right and others look sunk: they're just at different points in the same up-and-down cycle.

Fixed by measuring the floor from the TOP of the bob instead of from wherever the bat currently is, and by hanging the whole bob one full swing above the ground so no part of it can dip into the rock any more.

Confirmed and fixed. Flying monsters bob up and down around a centre point, and there's a guard that stops them entering the rock — but that guard was asking 'what floor is below me?' using the position the bat was ABOUT to move to. Once the bob had already carried it under the cave floor, the floor no longer counted as 'below', the guard found nothing, and it never pushed the bat back up. So the Stirges spent the whole downswing buried in the ground, which is what your screenshot shows. The guard now measures from the top of the bob, so it always sees the real floor and the bats stay above it.