I have noticed a couple times that when I book a United flight with one of the legs operated by ANA, the website lets me select my ANA seats but they don't show up in the UA Itinerary. When I go to ANA and use my PNR to locate my flights, the seat selection shows up there as selected. I think this is just a minor website bug on united.com. I just checked my flight for Sept 5th and sure enough, my seats were showing on ANA but not on united.com. Just my .02