XXX-ATL-ICN-YYY. Waitlisted GUC on ATL-ICN. It cleared. Called to apply GUC to XXX-ATL; not available, waitlisted. A couple of days later, flight showed as ATL-YYY on the app, missing the first segment entirely. A few hours later, the web site showed the same. I phoned in, they said everything looked OK. Next time I checked, the segment was back with the GUC cleared.