2017 Transit Connect XLT 2.5L w/115,000 and I'm having similar issues.
Was climbing a long mountain stretch of freeway when the car would not drop out of 5th gear. Moment later I got alert TRANSMISSION FAULT SERVICE NOW. I turned it around and took it home. This was a few days ago; the car shifts perfectly in and around town, even up short steep hills.
I am going to check for an overfilled trans first thing. Thank you for posting your account of transmission issues, glad that you found simple resolution.
