Depends on what the treatment was, I suppose.
Did you get a warranty/guarantee for the work? Without anything solid like that I would reckon you're just dealing with one of the foibles/realities of an older (but magnificent) motor car.
Was it a full, cut out and grind, eradicate all possible traces of rust (inside and outside the arch), right back to clean, shiny metal...........and then full, careful, methodical treatment, undercoats, top coats, bitumen stuff underneath, lacquer on top? And afterwards, over the years, a careful maintenance, cleaning regime, looking out for stone chips, rubbing of trim, cracking etc.?
Even if it was, I think by 5 yrs I'd have to accept the feasibility of a weakness being exposed. Maybe the bumper or fixings rubbing the paintwork?