Two things.
1. The more opaque glass is, the more energy is deposited in the glass. Etched glass will heat up considerably more than transparent or clear glass.
2. I do not recommend using calandered vinyl for most outdoor applications. Big flat sheets may give you a few years service, but individually cut letters and shapes have internal tensions as a result of the manufacturing process. The coefficient of the energy in heated glass in combination with the latent energy inherent in calendered vinyl will cause these results and other undesirable effects. In addition, IJ 40c is designed to be repositionable and easy to remove. It would make sense that the failure would be at the adhesive interface.
IJ 40c is a great product when used appropriately. A cast, solid color vinyl would have been the best choice. A cast, printed and laminated vinyl with a matching cast laminate would be a distant second choice (only if a satisfactory solid cast vinyl color could not be selected).