Zombies can break wooden doors even if out of range. For example, when a zombie is no longer at the door, the cracking animation and sound will keep playing for a few seconds. If the zombie has well progressed into breaking the door, it will break even if out of range. If the zombie was taken out of range earlier, the door will keep cracking for a few seconds then return to normal.

Steps to reproduce:

1- Build a house with a wooden door.

2-Place a "bait" inside (villager or player).

3-Dig a hole in front of the door (4 blocks deep is good) and cover it with redstone controlled trapdoors.

4-With the trapdoors closed, let the zombie begin breaking the door.

5-Open the trapdoor and let the zombie fall.

You will notice that the door breaking won't stop until a few more seconds, potentially breaking it.