Placing a slab to stop the water flow doesn't remove the water

How to reproduce

1. Place a waterlogged slab surrounded by blocks (2018-07-10_17.01.06.png)
2. Remove the slab (2018-07-10_17.01.09.png)
3. Place it again
   → The flowing water below the slab remains (2018-07-10_17.01.12.png)