To reproduce: In hard difficulty, find a zombie which can break down doors. Switch to easy or normal mode, put a villager behind a wooden door and watch the zombie break down the door.
Alternatively, such a zombie can be summoned using
/summon Zombie ~ ~ ~ {CanBreakDoors:1b}According to Jeb, zombies should only be able to do this on hard difficulty: https://twitter.com/jeb_/status/167274722072084480
what part is a bug? Zombies can break doors on normal or hard. that's intended. Pointing to a 2 year old twitter post doesn't make it less so.
If it did make a deifference, then we'd all be flying around on red dragons by now.