Confirmed and fixed. v83 runs flying mobs through the same terrain limiter as walking ones - the client's physics calls the foothold limiter for FLYING exactly as it does for NORMAL - so a bat cannot enter the rock. Ours skipped the ground code entirely for flyers and just bobbed them around their spawn height regardless of what the map does underneath, so they drifted into the cave floor and wall. Flyers are now held above the surface below them.

(On the dev branch - it will be in the next published build, not v32.)