If you were to spawn an enderman on an iron bar or any other block with a small surface, it can have a chance to teleport into the corner of the block as if it had the surface of a full block like a grass block. This can lead to it falling off which I doubt is intended. You can replicate this if you spawn an enderman on an iron bar and have nowhere else for it to teleport to, you can then fire an arrow or two on top of the enderman and wait to see it teleport onto the supposed corner of the iron bar and fall off; like the picture I had taken of it happening.