Zombies will stop being hostile if they can't keep getting closer to their target

That means if they're right beside their target (you, a villager), they will just stay there without attacking, until, due to any cause, they can keep chasing (a big enough gap is made between the zombie and its target), in which case they will attack again.

You can tell by their arms position that they're not attacking; they're not interested in you. Thus, they may walk away from you (fourth pic) if you stay still in a corner, and then regain aggro.

I suppose this may happen with other mobs, but I haven't tested it.