The bug
When attempting to load an old-ish superflat world or adventure map, a warning appears that says “This update makes your world both higher and deeper. It adds blocks and caves under your existing world, so there’s even more to explore underground.” This is incorrect because these worlds do not become higher or deeper, and do not get new caves underground.
How to reproduce
Download an old superflat world
Join it
Expected result
It just loads, or shows an accurate and correct warning.
Observed result
It displays an incorrect warning about underground caves.
This is because after caves and cliffs update 1.18 all terrain generation was rewritten meaning there was a high chance that many worlds could break. It’s just warning players that there could be problems when updating as this is the default message when creating a world before 1.18 and joining it after 1.18