Custom features not properly waterlogged

I've been working on a custom datapack to add the dripleaf plants to the mangrove swamp biome and have encountered a setback. I would like the plants to be able to generate in the water, but it seems they aren't consistently waterlogged. Now, I don't know if I'm doing something wrong (I don't really know what I'm doing), there's some hard-coded reason for this, or this is a bug. Any time I use /placefeature, it seems to work fine and properly waterloggs them, so I'm wondering if it's something to do with the way they initially load.