Summary:
The ceilings and huge chunks of soul sand valleys and basalt deltas are mostly netherrack now and, not covered with soul sand/soil or basalts, unlike in Java Edition.

Steps to Reproduce:

1. Go to the Nether and find a soul sand valley and basalt delta
2. Look at the ceiling and the lower part of every chunk in the biomes
3. Compare it to the Java Edition biomes

Observed Results:
The ceiling and huge chunks of the biomes are mostly composed of Netherrack

Expected Results:
The ceiling must be covered with soul sand/soil or basalts, as well as the huge chunks. Only minimal existence of netherrack should be there.

Original Description:
Have you noticed that soul sand valleys and basalt delta in Bedrock is out of place? Well it is, because the roof is covered with nether rack, not soul sand or basalts

Supposedly, the roof is covered with soul sand/basalts, so the biomes doesn’t look like it has the same terrain structure as the old nether, or it is just replacing the floor of the old nether. This exists in Java, the one that has the development version fist, but not in Bedrock.

The developers aim to have the same features as possible for this update. Thus, this is definitely a bug.