NOTE: I did search this issue, there were multiple things similar to this, but were most very unorganized, confusing and old or just not similar. I will try to make this as detailed as possible, I have tested this multiple times.
This is in Normal difficulty:
A Zombie will try to break down your door in Normal difficulty. He will never succeed, but when he finishes (or when he would) "breaking" your door, he will just completely lose interest with you. In Normal it seems as if he actually can break your door, but he can't. The door doesn't glitch by going away and coming back, it just loses the breaking texture that Zombies make.