Happening for me as well.
I can confirm that 21w44a has fixed 256 tall nether generation with an updated version of my data pack. Soul sand/soil, basalt deltas, crimson nylium, and warped nylium all generate up to the custom height limit,
Still occurs in 21w43a.
I attached the latest version of my 256 tall nether data pack updated for 21w43a.
This still occurs in the latest 1.18 snapshots including 21w42a. I created a datapack to make the nether 256 tall and yet those three surface builders refuse to generate their respective blocks above y=128.
I can confirm this issue still occurs on 21w40a. I really hope this gets fixed as I want that higher challenge when playing with friends.
MC 1.5.2 does fix this specific issue.
I managed to do a quick fix in Forge to fix the issue.
https://github.com/MinecraftForge/MinecraftForge/issues/513
Apparently Optifine has managed to fix this issue. Come on Mojang, stop messing around with new content and fix this!
Still occurs in 1.18 pre 5.
@Etlam Why is that especially strange? You're literally just describing the bug which is that chunk inhabited time is ignored in multiplayer.