This was a bug that I've discovered back in Ver. 1.16.0.60 (the beta version prior to this one) so I'm uncertain if this was fixed. However if it wasn't fixed... here's a brief explanation—I've discovered that you are able to clip though half slabs and stairs if you walk towards them with another half slab at the bottom, the hitbox is still present, however, it's odd how it works; you're able to walk on them just perfectly, however once you clip through them... the hitbox seems to move slightly upwards preventing the player from clipping upwards when flying or jumping—like a block above the slab. Not sure if this was fixed however... or if the bug is still present—I've ment to report it sooner, I apologize! (I was using a beta text hider in the photo—which hides the beta text above the screen).