If a zombie sees a villager he will try to follow it even after blocking the pathfinding AND blocking the line of sight. See images for proof of concept and how to replicate

1. Put a zombie in one chamber and a villager in the other (order doesn't matter)
2. Block them off completely
3. Put another villager next to the zombie and watch him not attack

In order to fix the Zombie's AI I have to reload the save. This makes it so U-shaped trading halls with zombie converters are really annoying to deal with (Trading hall example in screenshot: Zombie looks past villager right next to him because of another villager he can't get to)

Edit: Duplicate of MC-12689 Altough my example is more extreme as the zombie can't even see the villager he's locked on to