I posted about this before but couldn't find it. It's not only related to 2018. The problem is that at exterior openings, when you have the exterior finishes extending past the frame of the opening as is often the case with masonry, the edge of the exposed material shows up as the material used in the top exterior trim instead of the actual wall material.
Tech support after a few test was able to recreate the problem. The thing is that if you use any brick option for the header trim material, then the sides of the exposed materials show up correctly but if you use any of the other options (cast, stucco, ...), it shows the header material used.
With the brick option, the sides work correctly but the underside of the header doesn't really work properly but is less noticable unless you have a contrasting color or finish.