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
Put a zombie in one chamber and a villager in the other (order doesn't matter)
Block them off completely
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
Duplicates MC-12689