I had the same issue on a purely UA-operated itinerary booked with Lifemiles. I was issued a 134 ticket from Lifemiles, but when I looked up the reservation on the United website using both the Avianca PNR and United PNR, the United website said the reservation had no active flight segments. However, the Avianca website says the booking is confirmed and ticketed and so did the Avianca phone agents, so I thought I was SOL. I was worried that if I show up to the airport the day of departure, United may not let me check-in since they never received a ticket number from Avianca. Then a day later, a blessing in disguise came in. On the Avianca website, it now said that one of the United segments was "cancelled" which didn't make sense because the segment was still on the United schedule. That weird "cancelled" segment forced Avianca to reissue the ticket and this time I got a 202 ticket (from their sister airline TACA), that finally the United website was able to see as confirmed and ticketed. I don't know why the original 134 ticket wasn't recognized by United while the 202 ticket was. Maybe it was a different agent that did the reissue correctly. Either way, I'm relieved. Without that "cancelled" segment, I don't think Avianca would've moved an inch, as their incompetent phone agents will probably keep saying it's valid and ticketed on Avianca's end so there's nothing for them to fix.
Just wanted to add that though the ticket numbers changed, the PNR codes are still the same, so I'm hoping they "cleaned up" the previous PNR details and that nothing weird happens during check-in a few months from now.

Also FWIW, the original booking errored out right before the final checkout page on Lifemiles, so I had to call them and they manually made the booking and pay the taxes. So honestly, this whole issue probably wouldn't have happened if the Lifemiles website just did it's job.