Why do I have a Horizon between my First and Second Floors?
The Auto Horizon in 3D is positioned relative to the sill plate or structural slab; the higher of
the two. If the horizon appears at the second floor plan a sill plate has probably been drawn on that floor and
should be removed from the Floor System of the Second Floor plan. Another option is to turn off the Auto Horizon
while in 3D. Go to Background > Auto Horizon. Unselect the option and regenerate the model.