Steps to reproduce:

       1. Build a one-block-high space like the one in the video, and seal it with blocks (at least there are blocks within 4 blocks around, you can refer to the video)
       2. Use Creeper _spawn_egg to generate Creeper in a high space. You will find that Creeper will be generated next to the player or even under the ground.

Observed Results:
Creeper does not spawn normally, but spawns at the same height as the player who spawned him, next to the player.

Expected Results:
Creeper spawns in a block of space or not at all.