Intersection Priority Slabs

[EH21]

I have a basement composite slab showing gravel beneath intersecting with a footing slab. The intersection priority works fine between the gravel and the footing concrete, where the concrete takes priority, but it is leaving me with this vertical line running down. It has nothing to do with the wall above, as if I move the basement slab, the vertical line moves with it. The slabs are on two different layers but are in the same intersection group (so that intersection priority works) What is causing this? See screenshot below. 

[EH21]

Update: I did figure out the answer. If anybody else is having this problem, it is controlled through the Composites settings. This box has to be unchecked.