There are two systems used to maintain seat inventory. Apollo, the reservation system, and ACI which is used at the airport with 24 hours of the flight. They are supposed to exchange info but apparently once you’ve checked in it does not. In the past if you changed seat assignment after check-in from the "My itineraries" page it would look like the change took effect when in fact it did not. I will try to remember to retest this on my trip this week.