It’s a round box, fit into a round hole. Pretty snug all the way around, and insulation is stuffed all around it. I’m pretty confident it’s sealed up. That was my first thought. I added some insulation on top in the area where the frost was, but still left the soffit area open so air can pass through. Don’t know if that’s gonna help or notHave you sealed all the holes on the fan box in the attic? Including where the box meets the ceiling drywall? Likely it's mounted against a joist, and the slit between the two can let a lot of air out if not sealed properly. Silicone would do this (safer than foam that may expand to live wires inside the box).
I caulked all the holes (tabs pressed in to keep the thing in one box shape, as well as the grommet where the power comes in, AND the line where the box meets the drywall as well as where it meets the joist - as you can't reach where it meets the drywall there).