When placing stairs on the middle of the back of other stairs (for quartz stairs place on the darkest pixels on the back) the placed stairs will either not place, place on the side above where you attempt to place them, or place on the side of the stairs you are placing on.

Steps to reproduce:
1. Load a world and place stairs
2. Walk to the back of the stairs
3. Place stairs while aiming at the center of the already placed stairs
EDIT: If the originally placed stairs are facing west the placement is not bugged

(The pictures show first where to look and second where the stairs where placed.)