I have encountered similar problem on several European prototype models which had gold lettering, striping, or builder's plates. It seems that there is some sort of reaction between the metallic particles and some substance on the surface of the model (probably salt in human sweat left by touching the car, or possibly something in the ambient air) which dissolves (or maybe oxidizes) the metallic particles. All that is left is the clear binder used in that gold paint/ink. Usually, this is a slow process (takes few years).
I don't know of any way of fixing this. I suspect that if an overspray of clear lacquer (like Dullcote or Glosscote) was to be applied to the car when new, it would have protected the gold paint/ink from deteriorating. But who can predict that this will happen (since not all gold finishes are affected in this way).