Let me edit the post for simplicity.
I went through many steps trying to figure out what was going on, but here's the final way I fixed it. There was no problem at the arch top. The problem is where the two sashes meet. they meet on a single edge, one edge has four faces. To fix I push-pulled the bottom of the top sash down slightly (in reality sashes lap more than that. I erased the face to look inside, selected the new face between the sashes plus the edges (double click), group and delete (be sure any left over edges are gone). Then draw an edge to bring back the top sash bottom face.
Couple comments: Sashes overlap for the most part, you see one lock rail primarily, not both rails. Muntins are usually not as thick as the sash, they're recessed from the face, both sides. Vinyl windows may not have much recess. Sorry for the size of the picture, only good shot I could find.
wooden-win800.jpg