Bathroom fan leak in winter (poor attic ventilation?) 

(added comment Aug. 27th, 2021.
I think it could be caused by vent seal has failed at the roof. Need to ask a roofer to seal the vent pipe where it pokes through the shingles. The seal breaks down over time, and it's time to re-do it.)
We had -30 degree cold weather for a whole week, and then a strong Chinook hit Calgary, temperature warmed up to above zero within 48 hours.
All the condensation in the attic is finding its way to comedown, and vent opening become a drain hole for the attic. Luckily, we don't have such big temperature swing on regular basis.
The lesson learned here is trying not to over humidify your house when it's very cold outside.