It usually happens when climbable characters attach to wall that has no thok barrier.
I did a little level a while ago that arrows are self-explanatory. Walls that have modern like walls contain thok barrier (or what you wanna call it), however cracked walls doesn't have it at all same goes for orange ACZ wall that contains it and brown ACZ wall that doesn't have it.
and here's some gifs and DL link for someone if you wanna test it.:
- normal wall
- cracked (no thok barrier) / modern (with thok barrier)
- same as above
- brown (no thok barrier) / orange (with thok barrier)
I had a very good fix for not being able to climb on one-sided walls and then it turned out it meant Knuckles could climb in space in OpenGL, because OpenGL is so terrible it actually mutates some map data beyond comprehension. The fix to THAT was pretty hastily made in the run-up to 2.1.16 and failed to account for every eventuality, and that's how this bug managed to make its way into a release.
It's something I really want to dig into at some point but I don't have a fix right now, and don't expect one any time in the near future.