Optimising your world will change the nether biomes you had preveously in your snapshot

I had a warped forest, Now it's a basalt delta.

The evidence of the warped forest was still there but the particles, sound and biome data were erased and now it says basalt delta instead of warped forest.

To reproduce:

just take an old world from the 1.16 release and optimise it in the current snapshot. you'll see that the nether biome you're in changes, but evidence of the previous biome is still there