You gain winter attrition even when in your own lands and far beneath the supply limit of provinces, I do believe they had invented fire, houses, tents, warm soup, blankets and fur clothes during the classical era so feels kinda stupid that your manpower bleeds dry in your own lands if you happen to be in a mountaineous area or far north, its not like humans have adapted to the climate they live in or anything. -.-

no its not a bug... winter camping out is a risk even today

I kinda understand what you are saying. It seems absurd to lose an entire nations manpower in a single summer offensive because of attrition. I would love to see their evidence to support having 80,000 attrition casualities in a battle involving 90,000 people.

Get that, should at least be a troop command "permanent winter lodgings" that reduce morale by 80% that you can set while at peace for when they are not really truly "winter camping". :)