When you travel up and down a floor, you land on the other side of the stairs. However, if you turn while you're on the stairs in this way, you end up moving back on the stairs instead of just turning. I think this has to do with the game not putting you one more step ahead (so that you're
off the stairs), and the game thinks "player is on stairs, move play to next/previous floor".
Don't know if this makes a difference but I have my controls remapped so A/D turns and Q/E strafes.
Steam version 1.1.3. Image shows my party moving sideways on stairs. It is relatively spoiler-free. Imgur automatically converted to JPG so sorry for the loss of quality.