Zombies cannot break doors on Hard difficulty

I built a simple house with a Dark Oak door and put a villager inside it. Then I set the difficulty to Hard and spawned a zombie. The zombie went toward the door but no cracks appeared in the door, the sound effect of banging on the door did not play, and the door did not break after ~30 sec of this happening. I also tried with the other new doors and the same thing happened. Then I tried it with the old wooden door and the same thing happened again.