Basalt Delta ceilings are not made of basalt

The ceilings of the new basalt deltas biome are made from netherrack and not from Basalt, unlike in java edition.

Steps to reproduce:
1.Enter the nether on a new world
2.Locate a basalt deltas biome
3.Look at the ceiling and observe it is made of netherrack and not basalt as it should.