I have an even more frustrating story:

I was on a paid F from BOS to LHR with a colleague, different PNR's, but we had the PNR's linked. I was arriving into BOS from EWR and he was arriving into BOS from DCA. When I made the reservations the beginning of January, the seats were together. A week before the flight, the seats were changed (same equipment), and I had the ExecPlat desk move us back together. The night before, once again, the seats were moved, called the ExecPlat desk to put us back together, still same equipment. The morning of, same issue, had them put us back together. At check-in, once again, the seats were moved, had them put us back together. PLUS, his flight from DCA was an hour later then mine, so he checked in an hour after me, and when he went to check-in, once again, the seats were moved in that hour between my check-in and his check-in.. Finally had the BOS AC put us back together. For some reason, AA was determined for us not to sit together. Equipment never changed, F was not full, for some reason they refused to seat us together, and we ended up getting the original seats I had originally booked.