Sharing a very frustrating experience with PP upgrade. My mom was travelling with my 5yr old PVG to SFO in a paid premium plus cabin. both waitlisted for Polaris using my 1k pp. They are on different PNR but I called in advance to link the 2, no problem there.

12 hours before departure, there were 20 available Polaris and they were 8 and 11 on the Polaris list. As check in opened, people ahead of them started getting cleared into PZ. They could not check in online due to visa verification needs. I thought: fine, just need to wait until check in.

What happened next was beyond frustrating: people below them started to get cleared 1 by 1, skipping them. When they checked in, there were only 6 seats left in PZ, and check in agent told them they were 1 and 2 on the list but need to wait until gate to get cleared. when they got to the gate, the remaining 6 seats had been cleared to other people behind them, some from economy class (as they show up in waitlisting for R as well), so while they are still top of the list, they did not end up getting cleared for a 13 hour flight despite being higher on the priority list.

I have a hard time believing that 10+ people below them all paid for cash upgrade last minutes.(they were on the upgrade list to begin with so they were using instruments). I'm convinced there was an error or glitch that led to them being skipped by about ~10 positions. My theory is that because they have separate PNR but somehow linked together, the system could not upgrade them separately (because a 5yr old had to be seated with an adult in the same class).

Question:
- what else could have happened?
- what recourse do I have at this point as this is clearly a failure of applying the upgrade priority on the United side.