I think I finally fixed this.
The culprit is ACARS.php, with the line:
$flight->route_details = NavData::parseRoute($flight->route);
parseRoute actually needs several items from that route (lat, lng, and route). The following worked for me:
$schedule = SchedulesData::getSchedulebyFlight(substr($flight->flightnum,0,3),substr($flight->flightnum,3));
$flight->route_details = NavData::parseRoute($schedule);
Now to figure out why it calls the clearPreviousMarkers() a few seconds after it displays.