All mobs recognize bottom half slabs as air blocks and therefore
- can't pathfind on them if there are no blocks below them
- try to pathfind through them when they can't
- Create a void world
- Replace the starting platform by either lower slabs or air
- Build the test setup from the screenshots 2018-08-25_11.23.49.png and/or 2018-08-25_11.17.58.png (armor stand marks player position)
- Go to where the armor stand is located in the screenshots above
- Switch to survival mode
In the first setup, the zombie will only be able to walk towards the player until the stone blocks end. In the second setup, the zombie will take the path that goes straight towards the player and gets stuck in the stone slab.
There are a few videos attached to this report which also showcase this issue.
Mobs wont pathfind on only slabs when there is a block underneath its fine like in clip 1.
When there is a gap they will only pathfind when the blocks underneath them are connected to the blocks they want to go(clip 3).
I think they don't see slabs as blocks they can pathfind over, they think they fall through them.