Soul Sand and Soul Soil generate at the bottom of the Nether

The Bug:
Soul Sand and Soul Soil can generate at the bottom of the Nether.

Steps to Reproduce:

• Go below the bottom of the nether:

  /execute in minecraft:the_nether run tp @s ~ -10 ~

• Go to Soul Sand Valley:

  /locate biome minecraft:soul_sand_valley

• Run this command to remove the bedrock:

  /fill ~5 0 ~5 ~-5 127 ~-5 air replace bedrock

   ❌ You will notice that above the bedrock there was Soul Sand and Soul Soil.

Relates to MC-238930

Expected Result:
Netherrack would generate at the bottom of the Nether.