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.
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I believe this is actually intended.

Do you have any proof whatsoever of that?

No longer affects sea pickles.
Can confirm in 21w06a.
Can confirm in 1.17.1.
Can confirm in 1.19.

Can confirm in 1.19.3, also works with conduit

Can confirm in 1.19.4 and 23w17a
Confirmed for 1.13.1.