In a standard kelp farm setup,  As kelp is broken by pistons and floats to the surface, it will sometimes get launched at a specific angle and get stuck under the hoppers/ occupy the same block. Hoppers in the photos are not full, the kelp simply can't make it to the surface to be taken up by the hoppers. As this happens over time a lot of kelp is held up and has to despawn. Is a hopper supposed to be a full block that won't allow items to get stuck in the small open spaces under the hopper block? If so it seems its behavior is different in contact with water.