So I have no clue if this is a glitch, or simply just a design flaw when making the texture for this block.

What I expected to happen was...:

the texture would be the same as it is when it's a normal stair.

What actually happened was...:
When I placed a corner stair, it had no border on the back/bottom of it.

Steps to Reproduce:
1. Place a normal stair (so either a right side up or an upside down stair) 
2. look at the bottom or the back of it and you'll see a border at the edge of them
3. Place a corner stair

4. look at the bottom or the back of it and you'll see no border this time