It looks like many of the Pokemon that are meant to spawn in the Nether are unable to spawn there, leaving only Pokemon that spawn at low Y levels. I saw other tickets reporting this bug but they didn't have specific reproduction steps. I tested this using Turtonator.

System Information:
Minecraft 1.16.5
Forge 36.2.34
New World / No Datapacks / No Mods Installed Other Than Pixelmon
World seed: -2448745246724700183

Bug: Pokemon that are meant to spawn in the Nether cannot.

Reproduction Steps: Run /checkspawn under the following conditions:
- Biome: minecraft:soul_sand_valley
- Y-Level: 43
- Standing on cobblestone

Expected: Turtonator should be in the list of Pokemon that can spawn, as this location meets the following conditions from turtonator.set.json:
- LocationType=land met because of the cobblestone
- Level 25-55 met because Y level is 43
- stringBiomes="hellish" met because soul sand valley is registered as hellish in the betterspawnerconfig
- antiCondition stringBiomes="byg:subzero_hypogeal" is met because soul sand valley isn't that biome

Result: Turtonator is not in the list of spawnable Pokemon. It looks like the spawnable Pokemon are more similar to the ones that would spawn in caves.