Zombie sometimes does not attack villager after Iron Door is opened between them.

I locked up a zombie and a villager. Between them is an iron door which seperates them. When I open the iron door, the zombie doesn't attack the villager, even when he should. But when I leave my MC world, then join back and open the iron door, the zombie attacks the villager. I want the zombie to attack the villager. I have to leave my world and then join back many times so that the zombie can attack the villager when I open the iron door.