The bug
I was exploring around with new biome blending and noticed a chunk formation underground, I went to it and it was all air below 0 and converted incorrectly. This is very rare to happen as I seen the other chunks are fine. I checked the logs are there are 3 errors that are all in the same minute, but this happened on my other world without any error in logs.
UPDATE: The cause of this issue is when caves are generated below old chunks, the chunk contains no bedrock so it won't attempt to place caves below this chunk since after testing around, the chunk must contain at least 1 block of bedrock at Y=0 in the world for it to fill the bottom with caves. The fix would be detecting if the chunk contains an block at Y=0 instead of detecting only Bedrock.

Steps to reproduce
1. Load a world from 1.17.1 or below on 1.18 Pre-release 5 (or load Below 0 world bug.zip)
2. Explore around next to the blended area (If on downloaded world do: /tp -1201 -9 -85)
3. You can notice missing chunks but It is rare.

Observed result
Empty chunks below 0

Expected behaviour
Chunks to generate like the other chunks around