Soul sand slowness applied when walking on slabs placed above the soul sand

The bug

When you have soul sand on the ground, and place a half slab on top (I put cobblestone) it continues to give the slowness effect that is applied when you would be walking on it.
I expected to walk at normal speed when walking on the half slab placed on soul sand. I walked slow.

How to reproduce

1. Place a block of soul sand
2. Place a slab on top
3. Walk on it