In all variants of these biomes, the cave pockets seem to be generated after trees and plants. This causes many cases of floating objects throughout these biomes.

Copying reply from subreddit for completeness:

It happens because the 'vanilla' biome decorator does not play well with custom caves. I would need to make single block bo3 objects of all of the flowers/mushrooms/grasses etc and have them check properly for ground underneath them. The only reason I haven't is because there aren't many custom cave biomes yet that suffer from it.