So…I don’t know if this is a bug or not as it doesn’t cause any errors.
When I build a full wall to hold water back, THEN break the land behind it, it leaks. But if I dig first, then build the wall, it doesn’t. Images to show what I mean:
The large wall as seen was built BEFORE the parts were dug out behind it and it “leaks”. But the small channels can be walled and broken, and they don’t.