Upside-down Stairs Not Transforming Correctly on Corners

When attempting to connect two perpendicular upside-down stairs at a corner, the block transformation does not appear to render correctly. Stairs placed right-side up are not affected. This issue does not appear in 1.8.8, nor any snapshot prior to 15w38b, and is still reproducible in 15w39a.

What I expected to happen was...:
When connecting two perpendicular upside-down stairs at a corner, the stair should render with the 1/4th "nub" towards the back, creating a seamless line that wraps around the corner. (See screenshot "Expected")

What actually happened was...:
When connecting two perpendicular upside-down stairs at a corner, the stairs render with the 1/4th "nub" towards either the left or right side relative to where the user clicked. (See screenshot "Actual Left" and/or "Actual RIght").

Steps to Reproduce:
1. Run Snapshot 15w39a
2. Plase two or more upside-down stairs perpendicular, leaving one empty space where they would intersect. (See screenshot "Setup")
3. Place an upside-down stair in the empty space by clicking on the upper portion of the stair block.
4. Observe the incorrect block transformation/rendering.