It's as simple as follows: Huge fungus, regular fungus, roots (both types for last 3), and the basalt delta's iconic spikes cease to generate. Weeping vines and ceiling vine clusters in the crimson forest generate fine. The soul sand valley is also unaffected.
How to reproduce:
1. Go to affected Nether biomes
2. You will see that the proper surface features no longer work.
I don't know what sort of change in generation would cause this, but the fact that twisting vines and weeping vine clusters generate fine is weird.