When exporting worlds from 1.17.1 to 1.18, Stone appears in negative Y levels instead of Deepslate

So, I was just testing 1.18 conversion from 1.17.1 worlds when I found out that there was no Deepslate below Y=0, and it REALLY BOTHERS me. 😕

 

Reproduce

• Create a world in 1.17.1
• Load the world in 21w44a
• Go in spectator mode under y=0

Observed Result

There is no deepslate

Expected Result

There would be deepslate