Flat roofs can still occasionally generate in underwater caves

The Bug:

Flat roofs can still occasionally generate in underwater caves.

Steps to Reproduce:

• Enter any of the following seeds and coordinates.

• Version: 21w44a
  Seed: -7123216856550990646
  Coordinates: /execute in minecraft:overworld run tp @s -1101.30 30.00 475.02 -238.32 -0.44

• World Type: Single Biome, Lush Caves
  Seed: 8009750494097954235
  Coordinates: /execute in minecraft:overworld run tp @s -1168.16 29.55 7102.72 -1783.91 -6.64

Observed Behavior:

Flat roofs can still occasionally generate in underwater caves.

Expected Behavior:

Flat roofs would not be able to generate in underwater caves. Instead, these roofs should alternate between y levels to result in a more natural feel and look.