The bug
Generating a world with a datapack allowing for negative Y values will not generate any terrain nor terrain features below Y=0, even if bedrock appears in negative Y values. This includes ores generated with height offset, which should theoretically generate below Y=0.
Linked issues
is duplicated by 2
relates to 1
Attachments
Comments 2
a different bug MC-215117 has been marked as a copy of this one when it is not
I added a custom world file, and as you can see caves etc don't generate under y = 0, it's blank stone. I doubt (as the op also said) that specifically fixing this issue is necessary since this part of generation is subject to change / fixed by the revamp to caves to be released in 1.17.