This is a side effect of MC-214781.
Perhaps "local lava levels" could be introduced to fix this?
Comments 4
Remove features from v1.13 (minecraft: underwater_cayon & minecraft: underwater_cave) to add features from v1.17... It must be an error :S. Erasing variety in the oceans to add variety in the caves is very irregular. The underwater ravine with magma blocks were a challenge for boat navigation, and the underwater caves were good access underground at sea.
Does this still happen in 21w10a? (As MC-214781 is fixed)
These types of caves no longer exist.
https://www.minecraft.net/en-us/article/minecraft-snapshot-21w06a