Don’t know if it’s the same cause, but I remember something similar happening to me last year.

Several hours before the flight, AA swapped a 20F A321 to a 16F A321 (or possibly to A320, I forget), and then a few minutes later right back to a 20 F A321, but that was enough to mess up everyones seat assignment.

Couldn’t re-select an open seat on the app. Had to get it fixed in the lounge during my connection.