The Bug:

Caves and aquifers can generate with harshly flat ceilings.

Here is an example:

Seed: 1428380131082779128
Coordinates: /execute in minecraft:overworld run tp @s 47836.75 14.08 61752.81 -120.45 -7.50

Steps to Reproduce:

1. Generate a world with the seed provided above and teleport to the given coordinates.
2. Look closely at the ceiling of the cave/aquifer.
3. Take note as to whether or not caves and aquifers can generate with harshly flat ceilings.

Observed Behavior:

Caves and aquifers can generate with harshly flat ceilings.

Expected Behavior:

Caves and aquifers would not be able to generate with harshly flat ceilings.