Add XF and XC to the list too

I've never had computer put a seat assignment after I clear the upgrade automatically.
So, I believe the reason you were not given C seat was that you were confirmed in business class without a seat assignment and got bump down to Y because UA oversold C cabin.

Definitely call and complain about, but in the mean time, don't forget to call (or use online) to assign a seat in advance next time you book a flight and upgrade.