The bug
You can place turtle eggs and sea pickles in a liquid (water or lava) and the liquid would be blocked by the turtle eggs or the sea pickles, respectively.
Related issues
is duplicated by
Attachments
Comments

Also happens with sea pickles, and with water.
Can confirm with sea pickles in 18w15a.
I can see how this is intended with water (like with signs), but you'd expect lava to destroy the blocks instead (unlike signs)... so not sure regarding that part being WAI.
I'm not sure this works as intended. If you dig a one level deep hole, then dig out underneath it to make a deeper pond or lake, all the water except the top layer is flowing water. Which means if you place a sea pickle or a turtle egg under the water in this circumstance it creates a block of air around them.
[media]
The turtle eggs side of this could possibly could be considered a duplicate of MC-125351.