The Bug
I completed making my Soul Sand Water Elevator for the new lower depths under my base and the Tropical Fish spawned in the bubble columns of my build, building up item entities when it flops out of water.
Reproduce
Find a Lush Cave
Make a water elevator with magma or soul sand in the biome
Wait
Observed Result
Tropical Fish spawned in the bubble columns
Expected Result
Tropical Fish wouldn't spawn in bubble columns
This also impacts 1.18.1. I also have a soul sand water elevator from the bedrock layer which goes through a Lush Cave biome. Tropical fish spawn in the bubble column and get brought up to the top of the water elevator.
I've attached multiple screenshots of the bottom, top & middle section that runs through the Lush cave biome.