When upgrading worlds from 1.17.1 to 21w44a the biomes, in the caves that generate below the already generated chunks, are different than the surface biomes. 

These new biomes are those which would generate if world was originally created in 21w44a.

My test -

1. Created one new world each in 1.17.1 and 21w44a with the same seed  - test1

2. At -2, ~ , 29 the biome is Snowy Plains in 1.17.1 and Forest in 21w44a.

3. Upgraded the 1.17.1 world to 21w44a. The biomes in this world at the same coords is Snowy Plains till y=1 and below that it is different (forest).

Steps to Reproduce - 

1. Make a World in 1.17.1 and open it. 
2. Open the World in 21w44a
3. Notice the caves generating below y=0, in chunks that were already generated in 1.17.1. The biome will be different than surface biome (till y=1).

Now 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 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).

My suggestion for solving this- The Biome which exists already should be straightaway stretched same till bottom of the world in existing chunks and then Lush caves and Dripstone cave biomes can be super-imposed.