This area (link is zoomed/panned to the problem area) of McLean is broken, and when I switch over to Edit mode the border line goes away (maybe it is the highway?). I’m not sure how to make the border connect at Haycock Road & Dulles Toll Road.
Been a while since I fixed a boundary, but think I did it correctly still: Changeset: 183419898 | OpenStreetMap
The highway was indeed being used as border. It’s easier to see in JOSM.
Thanks! That sorted it out.