Originally Posted by
WineCountryUA
This is standard.
You should checkin with the first carrier. UA BP will appear on the app, after you taken the flights prior to the UA flight.
My guess if you had done nothing, everything would have been fine. Worked fine for me just a few weeks ago.
As for the agent's trick, never have heard of it and it wasn't necessary,
Although the OP didn't quite phrase it this way, some people encountering this issue (e.g., me) would like to be able to use the app to checkin to the downline UA metal/UA flight number segments at T-24 to be offered buy-ups. I haven't found a way around this. Sure, you can wait until the airport checkin on the partner carrier, but at that point (in Europe, anyway) you are typically at T-3 and there may be no buyup seats available to offer.