With the setup in the attached image, when you align yourself between the red wool and walk into the corner of the slab, you step up to the trapdoor's height and hit the side of the slab before stepping up the slab (losing all your velocity on whichever axis is perpendicular to the side you collided with). This happens in both 1.21 and 1.21.1 and did not happen in 1.20.4. I believe it happens for every entity that can step.