My girlfriend and I are working on a village that was completely wiped out by mobs. We collected and saved over 10 villagers.

We tried a new way that involved iron doors as our way to keep the zombie villager locked away until we were going to cure him. He started knocking down the door and eventually it broke. Unless this does happen, then that's fine. To my knowledge though, iron doors can't be broken by mobs, even in hard/hardcore mode.

Like I said, if this isn't a bug, then that's fine. We'll do something else, but, I'm 99% sure that iron doors shouldn't be getting broken down like wooden doors.