Looks like you had a CO flight with a UA codeshare. I had a similar issue with a UA flight with a CO codeshare. UA itinerary TPA-IAH-DEN with a DEN-IAH-TPA return with the IAH-TPA/IAH-TPA legs a CO codeshare. The flights showed up on both UA and on CO with a separate CO conf #. On the return I did OLCI on ua.com but the CO leg had disappeared from the itinerary, although it was still showed up on co.com, but I couldn't do OLCI at co.com because the first leg was on UA. I'd been upgraded on the CO codeshare (I have no UA status) and that apparently made the CO leg disappear. So I had to get my CO BP at the airport.

Outbound I did OLCI at co.com and got BPs for both TPA-IAH and IAH-DEN. I had to change the CO-issued BPs for the UA flight at the IAH gate because the GA said their barcode reader wouldn't take CO-issued BPs. The major problem on that end was that even though I'd entered my CO OP # when booking the flight on ua.com and the flight showed up on co.com when I logged in to my OP account, my OP # did not attach to the itinerary. I didn't know that until I did OLCI for outbound and had no OP# or plat/gold status on the BPs (and thus may have missed an UG on the CO leg). So I called in, got my OP# attached to the itinerary, and as mentioned above, got the return CO leg upgraded.

So there's not seamless integration between the UA and CO systems yet.