Lush caves blocks / vegetation like moss, clay and dripleaf don't seem to generate on aquifers (or underwater areas in general). Some clay patches do generate, but those don't need adjacent air and can generate buried in caves.

Entire caves generated as aquifers might generate without any lush caves features. Sometimes you might find moss patches, but because there's a dry cave below the aquifer, which generates moss on the ceiling.

Observed result

In this case, lush caves features don't generate probably because the biome only decorates on exposed blocks, but doesn't check for water. This is not the case with dripstone caves (See MC-221748).

Expected result

Lush caves underwater would generate moss, clay, and dripleaf (as they can be waterlogged) consistently like how they generate in dry caves, replacing most of the stone.

How to reproduce

1. Generate a single biome world of lush caves.
2. Find an aquifer, the larger the better.
3. Notice how most of the terrain underwater is devoid of lush blocks.