Deepslate transition starts too high which does not match old chunks

Summary:
Deepslate transition starts too high which does not match old chunks. It must start at Y = 4, not Y = 8. Only then will the transition match the bedrock.

Steps to reproduce:

1. Create a world in 1.17.
2. Upgrade to the latest pre-release.
3. Compare the deepslate transition in new chunks and old chunks.

Observed results:
The deepslate transition in new chunks starts too high compared to the old chunks.

Expected results:
The deepslate transition will be the same in both old chunks and new ones.

Image: