If an old world is upgraded, the biomes in the old chunks are not copied when the new caves are generated. The biomes are copied above Y=256 but not below Y=0.
NOTE: This was originally reported as an incorrect debug screen, until I looked at the world save and figured out what was really going on.
Steps to reproduce:
1. Create a new world in an older version. (I used 1.6.4 and 1.15.2 and seed 1 for both.)
2. Save this world.
3. Load this world in the latest snapshot (21w44a).
4. Go into spectator mode and inspect the biomes in upgraded old chunks -> Below Y=0, the biomes are not consistent with the biomes at higher y levels.
Copying my comment from MC-240581 (Duplicate)
I suppose this can be intentional, so as to support the generation of lush caves and dripstone caves below existing chunks but this has the potential of ruining gameplay, like for example, living in mushroom biome will be ruined as now below y=1, there will not be mushroom biome (unless it exists at the same place in the new generation coincidently) and mobs could spawn there.
one way of solving this can be - The Biome which exists already should be straightaway stretched the same till bottom of the world in existing chunks and then Lush caves and Dripstone cave biomes can be super-imposed.