The bug
If you create a Caves type world in 21w43a, go to the top of the world and you'll see Nether rack on the top instead of stone or bedrock. The nether rack goes down by about 10 blocks from 118-128. The bottom of the world seems fine and bedrock and stone generates normally.

Steps to reproduce
1. Create a world with "Caves" generator type
2. Go to the top of world
3. There is nether rack on top

Expected behaviour
Generates with stone and bedrock