Block in front of upside-down stair lighting bug

When an upside down stair (with blocks on the top and sides) is placed with a slab in front, will create a lighting bug.

What I expected to happen was...:
No lighting bug, and a normal texture.

What actually happened was...:
A darkened block texture (see picture)

Steps to Reproduce:
1. Put a upside down stair.
2. Add blocks on the left and right
3. Place a block in front. The bug occurs.