For oak doors, the sky check is done by comparing the blocks in front of and behind the door. For all other types of wooden doors, the sky check is done by comparing the blocks on the east and west sides of the door, regardless of which direction the door is facing.

This can be readily reproduced in creative mode., and it also affects survival mode. As a proxy for observing whether the door is valid, place a villager a short distance from the door and switch to night. If the door is valid, the villager will almost immediately run toward the door. Otherwise the villager will not.