I think that humidity is the problem, and that’s going to be inevitable in self defrosting freezers, which do their defrosting by continuously varying the temperature. It would probably be a lot better to put the coffee in a constant temperature chest freezer.
i don’t freeze coffee at home, but we are on the road for weeks at a time. Since we’d be at the mercy of unknown coffees, I freeze it in measured quantities, ground, in vacuum seal bags. Ground, because it packs tightly with a lot less air space. Works great.