Right clicking items on stacked sacks stored in a backpack puts that item in all sacks in the stack. Example link: https://www.youtube.com/watch?v=ZMQLGQa1pN8

ideally this would be fixed in supplementaries by checking the stack of sacks that is being right clicked and don't allow stashing items if the stack has more than one sack (as other mods that allow stacking sacks can run into the same issue).
But there's also a nonStackableItems config in both backpacks and storage that can prevent them stacking even if the stack upgrades are present