Caves generating below existing chunks are made of Stone instead of Deepslate, when upgrading the world from 1.17.1

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, will be made of stone instead of deepslate