My understanding is that insulating inside of a window can lead to condensation problems on the outside (see answer to this question). I have been using insulating window film kits for a few years and have not noticed any damage like this? What are possible reasons for this? (Maybe I have simply not applied the film very well?)
Actually, that answer is talking about condensation on the inside of the window, between the glass and the fiberglass insulation.
But you didn't put up fiberglass insulation, which leaves an air gap next to the window. This is where the condensation happens.
On the other hand, you put up insulating film which sticks directly to the glass without an air gap. Thanks to the film, the inside surface doesn't get very cold so there is no (or very little) condensation.