Not sure about Aeroflot but I had the exact same issue on Air Europa. Because the ticket was code shared (i.e. AF ticket on UX metal) systems did not allow it. Had I booked directly on UX it would have been possible. Eventually I found someone who could do it, but it took a lot of effort (and was worth it!).
BTW FB Silver does not automatically give FREE exit row seats, but only a discounted price. As Plat I was entitled to the seat without charge.