When I break a plant in water (kelp, seagrass, etc.) it has a chance of creating what looks like a slightly darker water block with two of the breaking block holes in it, as if someone had started to break it and then paused it. Other times, it will only affect one surface of the block - more specifically, the surface facing the air - and won't have any holes. This has only happened to me with water blocks which are surrounded by air on at least one side. Having a trapdoor on the side exposed to air seems to still count. It still behaves like normal - it flows, I can't breathe - but the only way to get rid of it is if the plant grows into it (if I'm growing kelp) or if I place a block there and then destroy the block. When I place the block, the breaking block holes are still there until I start breaking it.