It looks like you’ve edited some of the theme files directly, which means that all your tweaks will be overwritten when updating the theme. It’s likely one of these theme changes that has affected the scrolling/fixed positioning.
The best way to make changes to a theme is to use a child theme, otherwise Here are some guides in case you haven’t made one before:
I spoke with the developer of the theme, who clarified what’s going on here.
In Fictive if the height of the sidebar contents is taller than the window height, the theme will automatically remove the fixed positioning so people can access the sidebar content – otherwise, it would remain hidden below the bottom of the screen edge. To see this in action, try removing your widgets temporarily and you’ll see that the sidebar is indeed fixed.