Mobs cannot pathfind properly on bottom slabs, farmland, dirt paths, and other blocks without a solid top face

The bug

Mobs cannot pathfind properly and thus move very quickly on blocks without a solid top face, especially if there is air or another block without a solid top face below.

This is known to affect the following blocks (list may be incomplete):

• Bottom slabs
• Farmland
• Dirt paths
• (Empty) end portal frames
• Enchanting tables
• Carpets above string
• Chests
• Campfires

It seems to be affecting all mobs, though it seems like the issue is best reproduced with skeletons due to their strafing.

How to reproduce

1. Create a "Redstone Ready" flatland world
2. Run (optional, but makes the issue easier to see)

   /fill ~ ~-2 ~ ~20 ~-12 ~20 air
   

3. Run

   /fill ~ ~-1 ~ ~20 ~-1 ~20 oak_slab
   

   (Or any other affected block)

4. Give yourself some skeleton spawn eggs
5. Switch into survival mode
6. Spawn a skeleton on top of the slabs
   → The skeleton moves really quickly

Example

Here is an example in video: https://youtu.be/rNsKKNCFxMM