I had a similar problem back in October. I had booked EZE-MCI via IAH on UA. A UA schedule change around 10/26 changed the EZE-IAH segment from 11/14 to 11/15. The reservation on UA's site showed the new 11/15 departure, but the IAH-MCI segment was still on 11/14. I figured their system hadn't made all the changes yet, so I waited a couple of days. When I went back to check it both segments were gone from the reservation. I called UA, who said Avianca canceled it. When I called Avianca they said it was UA's fault. Finally Avianca rebooked my flight for 11/18, because that was the only date that award flights were still available. I then called United and they saw the other reservation and then moved the 11/18 res back to 11/15. Probably easier for them to do that once there was a reservation with segments they could do something with.

I consider myself quite lucky that there were award reservations left, otherwise I would likely have been SOL.