Bedrock from converted old worlds gets replaced with deepslate

Steps to Reproduce:

1. Create a world without the Caves & Cliffs toggle turned on;
2. Convert it by enabling it after creation;
3. Go under y0 and observe the transition (preferably with commands).

Observed Results:
Bedrock gets replaced with deepslate instead of stone, resulting in a layer of deepslate around y0, a sudden return to stone, then a slow transition back to deepslate.

Expected Results:
Bedrock gets replaced with stone, which makes the transition smoother.

Screenshots/Videos attached: Yes

Notes: