@RetiredWingMan Thanks for looking. I just adjusted all the waypoints to be exactly on top of the MRA generated route line, although I don't think that impacted the missing route section.
For now, I've written it off to something wrong with the original downloaded map. As I mentioned, performing a fresh map download seems to clear the situation up. I'll be riding that route in full early next week. I want to see it run successful to have complete confidence in NEXT. The Garmin Basecamp/XT combo has its share of warts, but I've never experienced anything like this.