Was considering a new thread but seems relevant to this one.

Last week Mrs. B accidentally did a voluntary split using the app to checkin, and UA completely lost her connecting flight!

We originally had a one-way SFO-PVR nonstop but due to a cancellation we were re-booked on SFO-LAX-PVR with an overnight layover at LAX. The app would only issue mobile BP for SFO-LAX (apparently needed passport verification) so I checked myself+child (no split) and my wife made the mistake of splitting the rez at checkin (who wouldn't say "YES" when asked if they wanted to stay on the upgrade list.

We land at LAX and I go to the transfer counter to get BPs for everyone. Myself+child, no problem. Child #2, no problem. When they get to Mrs. B she is informed that her reservation has disappeared and they have rebooked her on LAX-DEN-PVR.

ARE YOU KIDDING ME?? HOW DOES THE SYSTEM LOSE A RESERVATION WHEN A PERSON IS IN THE MIDDLE OF THEIR JOURNEY??

Anyway the "supervisor" was eventually able to put Mrs. b back on the LAX-PVR flight in the same seat, so all is well that ends well. But I still don't understand how the system can just cancel you out in the middle of a trip when you have checked in and flew the first segment. Maybe her first BP didn't scan?

Ironically the staff also put her on a return flight PVR-SFO, apparently they must have thought the system lost that as well. Of course we no showed on that one...

The moral of the story here is always get your connecting BP as soon as you can ...