When a humanoid walks into a wall it will either attempt to walk up or walk down, by floating or sinking, when in reality it shouldn’t because it is walking into a wall. This behavior applies to both R15 and R6 rig types.
I suspect that the humanoid must be seeing a potential staircase so it begins to ascend or descend accordingly, but it shouldn’t because there is an undeniable wall in the way.
This bug has occurred for as long as I can remember, for the past few years at the very least. As for how often this occurs, it isn’t uncommon to experience this floating or sinking behavior because all it requires is this simple part configuration of a wall on top of a floor or vice versa.
Place file: HumanoidRepro.rbxl (32.6 KB)