The flat world generation presets have not been updated for the new world depth in 1.18 – for example, the "overworld" preset in 1.16 contains 64 layers, putting the ground level at y=64, same as the typical worldgen ground level.

In 1.18 and 1.19, this preset has not been changed, resulting in the ground level being y=0 instead.

Oddly, the "ocean world" has had 64 layers of deepslate added to it, so it behaves like in 1.16.  But the others have not.