Piston pushing a non-solid block (Chest, Slab, etc.) into water creates a ghost water block

Trying to waterlog a chest or a slab by pushing it with a piston doesn't work. The water appears to stay put, so I assume this is supposed to work, but breaking it makes the water disappear. This was getting in the way of magma bubble columns, which I wanted to use to make it easier to obtain a blue axolotl.