Mobs spawn in floor is space is not tall enough for their height.

If the ceiling height is less than 2 blocks, mobs that cannot be spawned unless the height of zombies etc. is 2 blocks or more may spawn with the mobs buried in the ground.

 

"Reproduction method"

1. Create a space with a height of 2 blocks from the ground to the ceiling.
2. Install a trapdoor on the ceiling to make the ground-to-ceiling height less than 2 blocks.
3. Allows monsters to spawn about 25 blocks away from the trapdoor-lined area.
4. Creepers and spiders should spawn in this state, but rarely monsters such as skeletons will spawn with their lower body buried in the ground.