Yqe

Seems like a bug with world generation and not world conversion. Are you sure this generates the same when using the seed?

Confirmed fixed in 1.18 pre release 5

Should be working as intended, since the game should is only supposed to generate new caves where there is not bedrock 

Or select if the generation should be done chunk wise or column wise, basically choosing if to use pre2 or pre1 method

The fix to this bug makes it so new terrain will generate underneat old terrain everywhere, except where there is no y0 bedrock in the entire chunk. I suggest this be reversed as this does not affect any worlds that have been generated in the last 10 years or so, and it would cause irreversible damage to all void holes that are not full chunks. 

The only bug this relates to was patched 11 years ago, this is only a consequence that if anyone wanted to fix they would have done themselves in their own worlds already.

This could be marked as "working as intended" as before it was fixed, the game generated new terrain under old terrain only where there was bedrock, as it should have. The fact that these old worlds didn't have bedrock in certain blocks shouldn't matter, that was a separate bug that has already been fixed.