(Quick disclaimer: I understand that this is an "experimental" feature and may not have continued support, however, I believe that it is worth mentioning regardless.)

When both the max world height and the bedrock ceiling are increased in the nether, the nether biomes do not generate as intended. Above y=128, the biomes' normal blocks do not fill in as they are meant to.

For example, in the soul sand valley, soul sand and soul soil clusters will not generate above y=128 (clusters, because some pieces can go about 3 blocks higher).

In both the warped and crimson forest biomes, neither the nylium nor warped trees generate above 128.

(It's not my place to guess, but) Above y=128, the noisemap seems to be the same, but the generation appears to follow the nether wastes generation pattern (default/fallback biome?)

Screenshots with co-ordinates as well as the world generation file are attached. (World generation file kindly generated and provided by u/MrDodojo over on their post here.)