In the nether, all biomes generate with the nether wastes generation settings, the mobs/structure spawning remains the same.

I discovered this bug in my survival world when I realized skeletons were spawning in the nether wastes, I then checked the F3 menu to see it was actually soul sand valley biome.

How to reproduce this bug:

1. Generate a regular world

2. Go to the nether

3. Use the /locatebiome command OR Fly around until you find a different biome with the F3 menu

4.There will be only netherrack, quartz ore and nethergold ore, but there will still be special mob spawns eg. enderman in warped forest

Note: After some testing I have observed a small crimson forest generate so this bug may not occur 100% of the time.