Put the summary of the bug you're having here

What I expected to happen was...:
I expecting to just create a normal cave mode world in minecraft 21w44a snapshot

What actually happened was...:
I created a world, but even if I set world biome to plains every block like stone or ores were replaced by netherrack. It was in overworld. However, structures like spawners or mineshafts normally have blocks of planks and fences. 

Steps to Reproduce:
1. I clicked "create new world"
2. I set game mode to survival and I allowed cheats. i also set world type to "caves"
3. The biome for this world was set to plains.