Cave vines can generate into water when starting above water and the length is larger than the distance to the water surface.
How to reproduce:
Due to MC-219200 seed and coordinates may be incorrect sometimes
The "allow_water" setting of the "minecraft:block_column" feature is behaving differently from expected: If set to false the block column can generate in air and water; if set to true the block column is generated independent on any blocks and generates through stone.
The Cave Vines wouldn't generate into water