The Bug:

Magma blocks can generate on the ceilings of caves below aquifers.

This is a result of having magma blocks sometimes generate at the bottom of underground bodies of water.

Here is an example:

World Type: Single Biome, Lush Caves
Seed: -6780154460659831019
Coordinates: /execute in minecraft:overworld run tp @s 7552.86 3.93 -379.46 -128.89 -47.10

Steps to Reproduce:

1. Generate a world with the world type and seed provided above and teleport to the given coordinates.
2. Look closely at the magma block(s).
3. Take note as to whether or not magma blocks can generate on the ceilings of caves below aquifers.

Observed Behavior:

Magma blocks can generate on the ceilings of caves below aquifers.

Expected Behavior:

Magma blocks would not be able to generate on the ceilings of caves below aquifers.