As of Snapshot 20w28a, Older nether saves have had their biomes scrambled, despite the actual terrain generation. This bug is very similar to the biome updates in older versions of the game, in which when the game updated, a jungle biome, for example would register as a desert biome, and would get all the properties of a desert biome, like the brown-ish grass, all without changing the terrain. The same thing is happening right now to older nether biomes. I have been playing in a nether save older than 1.16, and have updated the terrain and built in it before biomes were added/rearranged, so resetting the nether for me is out of the question.

below will be a screenshot example of the bug. The warped forest terrain was generated sometime before the snapshot that added piglin bastions. It now registers as a nether wastes biome. 

So far, this bug has only occured in 20w28a and upward. 20w27a does not update the biomes.