*Nylium no longer generate below the ceiling, but Soul Sand and Soul Soil still generating (and at the bottom as well)

The Bug:
Nylium, Soul Sand and Soul Soil can generate below the bedrock of the nether ceiling (and above the bedrock at the bottom)

Steps to Reproduce:

• Go over the nether ceiling:

  /execute in minecraft:the_nether run tp @s ~ 140 ~ 0 90
  

• Go to Soul Sand Valley:

  /locatebiome 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 under the bedrock there was Soul Sand, Soul Soil or Nylium.

• Go below the bottom of the nether:

  /execute in minecraft:the_nether run tp @s ~ -10 ~ 0 -90
  

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

21w41a and 21w42a: MC-238930.mp4, MC-238930-1.mp4

21w44a: MC-238930 soul sand.mp4

Relates to MC-171517 and MCPE-74607.

Expected Result:
Netherrack would generate below the nether ceiling (and above the bedrock at the bottom).