Pathfinding of mobs (i believe all mobs do that but i just tried few) gets broken when the mob falls down to water from a open trapdoor while walking towards a direction or to something (bed, workstation, food ect.). It doesn't matter if the "walking" is random or not. And the mob can't be stopped by anything other than a walkable block. Can't stop a villager by spawning a zombie in front of it, can't distract a zombie with a villager, can't make a cow follow you by holding a wheat. Even removing or destroying the thing that they were going towards can't stop them. They are going towards the direction that they decided to go before fallen into the water.
Update: I found out that it's not only trapdoor. It happens with open fence gates and it happens if you break or move the block underneath them. Quitting and reloading the world stops them.
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