Pavlo’s approach
I would start with analyzing user flow through the app. If Stories views increased but overall session duration dropped, it means that the drop presents in some other place of the app. Possibly users either spent more time in the previous version of the app or they staid longer before jumping into stories or stood somewhere after exiting the stories. The idea is to find that screen with the drop. After that we should ask what were the objectives of new stories feature and compare them to the current situation. According to that we can set the plan for further features adjustments.