Steps to reproduce

1. Build a floating loop of blocks.
2. Place a top slab somewhere on the ring of blocks.
3. Spawn a cave spider or silverfish on the opposite side of the top slab from the player.
4. Set gamemode to survival and hit the monster.

Expected result

It crawls under the slab and gets you.

Actual result

It walks all the way around the ring and gets you.

Cave spiders and silverfish (and probably endermites) do not consider going through a hole half a block high as the shortest path to their target, even though they are capable of passing through the hole.

In the video you can see how they do not pass through the slab although it is the shortest way.