Biomes in old chunks are not copied to new caves below Y=0 when chunks are extended

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.