Anyone ever had a passenger insist on taking the middle seat because that's what they were assigned?
Yes. But each time FAs explained the situation to them and they chose either the aisle or window so we could sit together.
Years ago my dad and I were flying on a 3+3 on a flight that was rarely more than half-full, so we chose aisle and window. There were maybe 20 people on the plane. Of course, the last person to get on was assigned to the middle seat and he was adamant about sitting there. FA saw the oddity of 3 people sitting next to each other on a flight with entire rows that were empty and told the guy he could take literally any other seat on the plane. I looked back later and saw him on an aisle that had someone on the window, even though the rows in front and behind were empty.