Can't place kelp or seagrass outside of natural water

I want to make an aquarium with kelp/seagrass. But when I try to place it in manually-placed water, the kelp/seagrass isn't waterlogged, breaking the kelp/seagrass. Oddly enough, you can only place ONE kelp with commands in this situation. If you try to place another directly on top, it will break both. I can't provide any visual evidence because I have no screen recorder. This does not affect Sea Pickles nor does it happen in naturally generated water.