Steps to Reproduce:
1. Generate a new world in 1.17.41.1 with the seed "-1957154863"
2. Teleport to the coordinates +7 +2 +4
3. Upgrade the world to 1.18.0

Observed Results:
Bedrock, that had not bedrock fully underneath all the way to Y=1 is still there.

Expected Results:
All Bedrock between Y=1 and Y=5 is replaced with deepslate.

This has been observed on Windows 10 and on Bedrock Dedicated Server and likely occurs on all platforms.