Minecraft 18w11a, Minecraft 18w16a
Summary: Moving a waterlogged block moves the water source with the block rather than leaving the water where it is. The inverse is also true, moving a dry block into a water source removes the water source rather than producing a waterlogged block.
Steps to reproduce: Place a waterlogged block such as an inverted slab, then move it with a piston or piston and slime block(s).
What should happen: The block should be moved out of the water source, leaving the water source behind.
What actually happens: The block and water source move together.
Commentary: This behavior is illogical, because if you use a piston to pull a waterlogged block up out of the water, the "natural" result would be to get a dry block above the water and leave the water behind. Likewise, if you push a waterloggable block into a water source block, one would expect it to become waterlogged.
It seems blocks retain their waterlogged state when moved. A possible fix would be for a moved block to check to see if the destination position is a water source (or another waterlogged block) or not, and change its waterlogged state to match that of the destination location.