Paul, I have seen this
I have seen this reaction before with aerosol spray–specifically when the aerosol was sprayed into a dish (to collect it) and then placed between (spectrum glass) white opal (base) and clear using small drops of hairspray applied with a toothpick. I do not know what brand of hairspray it was. The areas where the hairspray was applied turned brown and hazy. I have also seen hazy spots appear after using white school glue that did not totally dry before firing.
This has never happened to any peices I have tried using non-aerosol hairspray. All the instructors from whom I have learned have relayed the same suggestion. As far as the exact chemical reaction, I have no idea. Looking at both MSDS, non-aerosol can be found at:
There are some differences, but I am not a chemist.
Maybe it has something to do with it being between layers. The original question was about using frit. Maybe enough of the frit liquified first and trapped enough material underneath before it could all burn off. It is hard to know without more detail.