When kelp breaks, the items are supposed to float to the surface so they can be retrieved, but some of these sink down to the ground.
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Confirmed, but not often. Could be when part of item floats over an adjacent block that is not source block.
After a piston push, the water block in front of the piston no longer is a source block, but falling water. You would need to build in a way for the water sources to regenerate at the piston level.