Note that all apps are sandboxed on stock Android as well as on GrapheneOS, but putting them in separate user profiles does provide additional isolation.
GrapheneOS improves user profiles over stock Android in number of ways. One of those is the ability to logout of user profiles when you don't need them. That way Instagram will not keep running in the background.
Notification forwarding for a user profile is an optional feature. If you don't want notifications forwarded from a specific user profile then you can disable forwarding for that profile. If you've logged out of the user profile anyway, there will be nothing to forward, as the user profile is at rest.
Note that you can isolate apps further by utilizing the Vanadium sandbox when installing them as progressive web apps (PWAs). That's how I use Instagram and Twitter.