The Bug:

Nylium generates under the lava ocean in the Nether. This was covered in MCPE-74607, but that ticket got incomplete.

Steps to Reproduce:

1. Create a normal world with cheats and with the seed: 0
2. Go to spectator mode.
3. Run command: /execute in nether run tp 18 26 130
4. Look around until you see warped nylium under lava.
5. Now run command: /execute in nether run tp 627 24 -9
6. Look around until you see crimson nylium under lava.

Expected Results:

Nylium should not generate under lava oceans and should generate as netherrack instead.

Observed Results:

Both types of nylium can generate on lava ocean floors.