I was not, and just tested again. It turns out the dolphin survived for exactly two minutes. Whoops!
But it's not supposed to flow into any blocks other than air even if waterlogable? If it works as intended, how come water doesn't naturally flow into these blocks it logically should?
No. It only works on multiplayer though, not single-player.
EDIT: OK, now I can't even reproduce it. If a mod is reading this, you can mark it as resolved.
Fixed in 1.13-pre7
OK, well it's fixed for me anyway in 18w21b.
Let's maybe not fix this 'issue'?
I know, but that is the environment the bug occurred in.
I think this is WAI, you might want to destroy something with a minecart and TNT without actually removing the path of the minecart.