Create a water source block and make it flow to a side and then one block down. Then place coral, coral fan, kelp, sea grass, or sea pickle there. After that remove the source block and see that the placed block is now inside a new water source block.
As far as I tested this only happens to force-waterlogged blocks of sea plants as well as sea pickles and coral fans, but not optionally waterlogged blocks like fences or chests.
I think it may relate to MC-133354, but this is a bit different. See screenshots.