I'm not sure whether zombies are meant to be this stupid, but it appears that they miss an open door when chasing a player. Instead they attempt to break a closed door as shown in the attached image
MCPE-10949
Zombie door logics
Resolved
Fixed
0
(Unassigned)
Unconfirmed
Phone - Windows Phone - Other (Specify in description)
Lumia Cyan 3 (Application version: 15.3.54)
0.12.1
Future Release
Retrieved
Confirmed on Samsung Galaxy S4 android 5.0.1 running MCPE 0.12.1 The logic seems random - some zombies go through the open door others smash the closed one...