Does powdery mildew form on freshly cut leaves, within days at 70% humidity?
at 70% humidity???!!! Ummm....Yea??? Mold very easily can form that way. Even when curing bud you NEVER want your jar humidity to breech 65% as it can lead to molding. Aim for getting a balance humidity of (60-62% in your jars at least) when curing. But yeah dude....70% will cause molding of your bagged/bucketed leaves within days... were they your sugar leaves?
I dry my leaves so that they are all not over piled on top of each other. You don't want the leaves to be higher than a few inches. Paper is better to dry on than plastic. Also you should place a fan to blow on them as well. Turn the leaves at least once a day.
Your room may be 70% but your space inside the bucket containing the leaves may be be over 70%. It's all about air flow.