I have an old world that was originally created in Alpha v1.1.2_01. The latest version of this was played in 1.17.1. I started this version in 1.18 pre-release 1 to see the new caves beneath the old world and it was a mess. There are holes everywhere. They started at Y 0 and went down to -64 and through bedrock.

I checked it and it only affects the world originally generated in the Alpha because when you get another part of the world it was the way it should be.

The caves at 0 are in place and the bedrock at 0 is gone.