Do you leave the poly feeders on all year round, complete with syrup through the colder months?
Although I've put fondant directly onto the top of frames without any problems, I've heard it said locally that there's a risk of it dripping between the frames and encasing bees.
The bakers fondant we use comes in a thick blue polythene wrapper, we cut fondant and polythene at the same time because it's less messy, then rearrange the polythene so there's an access gap. They don't destroy this blue stuff, but they will waste time shredding clingfilm and taking it from the hive.