If you use the "caves" world type in the 21w44a snapshot and set the world biome to lush caves, the world is made out of netherrack but with lush caves decoration and water.

To reproduce: In the snapshot, make a caves world and set the biome to lush caves

Intended result: The world should look like a lush caves single biome world but with Nether terrain generation

Actual result: This weird Netherrack world.