The bug

Mobs cannot pathfind correctly in certain situations.
They would get stuck trying to pass through a <2 block gap.
Note that they can perform the same task correctly when the block is replaced with something with a full top, but still transparent (e.g. top slab).

Further testing suggested that sometimes this still happens even if the block is removed entirely.

To reproduce

1. Open the attached world MC-196548.zip and watch the behavior of the villager attempting to pathfind to a workstation.

Observed Result

The villager gets stuck attempting to pathfind between the stairs and the low ceiling.

Expected Result

The villager would recognize it as an invalid path and turn around.