Create a water source block and make it flow to a side and then one block down. Then place coral, coral fan, kelp or seagrass 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 coral 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.