Earlier this year I did EWR-HKG-SIN-SFO-DEN-EWR as a MR with 4 hour layover in SIN and the system treated it as a domestic miles+copay upgrade thinking it was EWR-SFO with 2 stops. Was able to find R space at booking so all the segments cleared right away - except for HKG-SIN which for some reason wouldn't let me upgrade. I knew well enough not to call and draw attention to it.