I found a bug in AC13 StairMaker, which occurs in following sequence:
1. Make a new stair with StairMaker, set Structure to far right button /simple concrete stair, save and place on plan.
2. Select the stair, edit it and set Structure to far left button /concrete with tread/, save and view in 3D. The bottom flight is missing in 3D, only the treads are visible.