I was testing mobs AI around fire and lava, when I noticed a bug in the hostile mob AI.

When I went around the corner, the zombies suddenly stopped, forgetting what they were chasing, literally the same second. 

I tested the same with Villagers too for the same effect. And this way you can test even better because this way you can see the both the zombie and the zombie's target at the same time.

How to reproduce

In survival, lead a zombie to follow you around a corner The zombie will stop instantly after you're out of his sight.

Or let a Villager lead the zombie. This works too.