If you go in spectator mode and go below the bottom of the ocean you'll see that all of the caves are pitch black unless there's lava nearby. If you go below an island or a nearby beach, you'll see that there is glow lichen in the caves beneath them.
This is most likely caused by the fix of MC-212113
I did some testing and it seems to be fixed in 20w40a, so I posted this for nothing, please close this