This is still an issue in 1.18.1 and is ruining my pig racing map. Please fix this ancient bug!
The new world height and cave generation is only available in the second part of the update (1.18), releasing in the holidays. To try the new generation in the current snapshots, you have to download a datapack.
This is not a bug, it's intentional.
This might be fake, or done on an older version. I tried recreating this bug by loading a world in the latest version with the seed 0, went to the specified coordinates, and there is nothing to be seen. Just normal terrain.
I created a video detailing this bug: https://youtu.be/J1QutazIIO4. The bug is very game-breaking, and genuinely needs to be fixed.