[MC-176390] Nether vegetation generates only on the highest possible place, lower places look barren and empty

In the Nether, when a platform generates floating above another platform, plants and fungi generate only on the higher platform and the lower platform only gets surface blocks. The issue is not visible in Nether Wastes or Soul Sand Valley, but in Crimson Forests and Warped Forests it's very easy to see. When flying through those forests, occasional barren places with only Nylium and Netherrack can be found.

It seems that Nylium prevents anything from generating below and that only higher Nylium platforms get vegetation. Vegetation still attempts to generate below (parts of) platforms that are not covered with Nylium. This causes weird, empty places in nether forests.