Dials are really fragile, as we know. If you see a pristine one, you can be pretty certain that the movement is in decent shape. But when this happens, the eyebrow has to go up:
They have never serviced it, because "it's Rolex, it will run forever, don't need to be serviced", perhaps.
My favourite 1950s vintage Omega bumper came to me in pieces, having been dismantled over 40 years ago. I have shown it on the MB many times. The dial on it is pristine, and all original. When I collected all the parts prior to returning it to health, most of the steel calendar components behind the dial were rusted. An odd occurrence for such damage to happen, with no damage to the dial.
Some medical conditions cause ammonia to come out in a persons sweat. I learned of this when my nephew developed a liver ailment. A very severe case of Diabetes can cause acetone to come out in the sweat. Could be something of this sort ate away at the seal over many years and allowed the caustic biochemical inside from the back. Ammonia can dissolve platings and non ferrous metals while not affecting the paint or noble metals of a dial.