I also track everything in a spreadsheet & have 1 or 2 FFC credit PNR's that do not show up in the future flight credit tab. But if I go to my "Cancelled" flights, the missing PNR is there & when I click on it & it shows a FFC like the other PNR's in the FFC tab. Not sure why?

Moral of the story. Track the stuff that is important to you. Especially if UA owes you something