When I tried to build a mob farm up in the sky above deep frozen ocean biome, I realized that skeletons or strays are not spawning in the farm or in any caves. Strays also spawn only at the surface level, not in the mob farm or in the cave.

How to reproduce:

1. Create a creative world
2. Teleport to deep frozen ocean biome
3. Create a simple mob farm with a roof for cave spawn, or explore caves

We should be able to see that no skeleton or stray spawn in any caves or any spawn platform above the ground level (There is also no skeleton spawning for surface spawn)

Expected result:

Skeletons should spawn in any biome except mushroom island, the same way as other hostile mobs.

I also attached the images in this issue as well. The first two images are the mob spawn as surface spawn. We can notice that no skeletons or stray spawns on the platform. The third image shows the cave spawn under the roof. We can see that no skeletons or stray spawns inside the cave.