One point is that what is today still is commonly called "zapon lacquer" and marketed as such, has frequently become an acrylate lacquer or varnish. Both may react differently.
Secondly, original old zapon lacquer is prone to yellowing and crackling under the direct influence of UV light, over the decades.
Thirdly, the "case colours" are actually very thin oxide layers, with a thickness of only a few nanometres. The thickness also determines the color (!). Hence, they are rather sensitive to any kind of mechanical influence or wear. The layer thickness of the zapon lacquer cover over them is probably thousandfold.
