Something between the exhaust vent and roof vent isn't insulated properly.
Warm surface, in a freezing cold attic space soon causes condensation that freezes, and will then thaw during the next warm spell. Builders are cheap as hell these days, and pretty sloppy in areas the consumer doesn't see.
The exhaust ducting may not have been attached to the roof vent at all, maybe there is no duct, wouldn't surprise me in the slightest.
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