Minecraft 15w38a, Minecraft 15w38b, Minecraft 15w39b, Minecraft 15w39c
Stairs will auto align when in placed in a corner so that they look nicer with other stairs placed next to them. At 15w38a the alignment no longer works for stairs that are upside-down. When placing the block attempts to align with other stairs but does so in an incorrect fashion.
After trying out old saves with corner upside-down stairs, I found that they too look out of place. So, as I understand it; it has to do with the rendering of the blockstate and not due to error in the placement code that chooses the blockstate.
After further testing. I found that the selection box (the block out line that appears when cursor is pointed at the block) is also behaving funnily. So I tested the collision of the block by firing arrows at it to determine better where the block starts and stops. The rendering/collision is indeed off. Arrows would collide on portions of the block that were rendered as "air" and pass through portions rendered as "block". After trying this out I found out all the upside down stairs now have collision error.
I found that any upside-down stairs (not just cornered ones) had collision errors.