Interesting... The only thing the defroster does is turn on the windshield deice, so I suspect Asobo considers the "melt system" (as it's called in the material parameters) to include the windshield deice, even though the windshield is a different material... I've made the appropriate change now and will see if it makes a difference. If you haven't noticed, the icing features in MSFS were not very well thought out, in my opinion.