Mobs patchfind doesn't recognize lower slabs as full block. They recognize them as air. This affect for example enderman (farms) and zombie, but also other mobs, like wolf chasing skeleton/sheep, villager running from zombie etc.
To reproduce:
First attempt: Make a pit around villager. Cover pit with lower slabs. Spawn zombie. Zombie won't get villager, cause it sees a pit around.
Second attempt: Make a 2-high hole in a wall around villager by making 3-high hole and upper slab on top and lower on bottom. Zombie will pathfind through it. In 1.12.2 It couldn't.
Third attempt: Make 1.5 high hole with slab on bottom of it. Zombie will pathfind through it and hit the wall (can't fit through). In 1.12.2 It would look for walkaround.

Found while testing: MC-134118
Probably this bug covers the mentioned one.