Water goes through waterlogged stairs and slabs' bottom part when being touched by flowing water

The Setup

1. Place a slab/stair (the bottom half) two blocks above the ground level;
2. Surround it with solid blocks;
3. Waterlog the slab/stair.

At this moment, there is a waterlogged slab/stair inside of a four sided cage to prevent water from flowing.

Reproducing the bug

1. Place water next to the setup one block above the ground level;
2. Let it flow below the slab/stair;
3. Remove the water source placed in step 1.

Expected results:
The water is properly removed.

Observed results:
The water is now vertically flowing from the slab/stair even though it's placed on the bottom part of the block.