Part of Stair becomes invisible when placing a block near it.

When placing stairs in a formation like so (http://i.imgur.com/LzDAQEC.png),
placing a solid block in front of the outer_right stair makes the top half become invisible from one side, causing the stair to look like a slab (http://i.imgur.com/2wMIdu5.png). The stairs look unchanged on the other side (http://i.imgur.com/4GNOmFn.png), thus creating a block that looks like a slab from one side and stairs on the other.