many mobs are spawning in incorrect places on blocks, where some spawn slightly above the block they should be and others are spawning slightly below (or inside) the block they are spawning on. Large slimes will fall through my ceiling because they spawned half inside the full block on the ceiling, and other mobs like creepers and zombies will rarely spawn on glass where the covered block below is an appropriate light level for them to spawn, as if the glass block was not there