Same thing happened to me recently. One of the segments seat assignments dropped off and showed unassigned. Trying to get a new assignment showed the same symptoms that you reported. Called AA and they said I still had the same seat assignment, but had no explanation as to why I couldn't see it online.
The only unique thing about it that I could see was that it was a multi-segment flight with the same flight number.... LPB-VVI-MIA.... all #922