Slabs which become waterlogged by having a piston push them into a water source block can't be drained by a dispenser with an empty bucket with it.
Additionally, when the slabs are pushed into the water source they have a strange white particle effect.
This is a duplicate of MCPE-178141 MCPE-171416 as pushing a slab into a water source will create a ghost block that cannot be interacted with, and "breaking" particles will be produced when a piston interacts with water.