Kinsy i dont use sandboxed GP so i am not super familiar. But it sounds like this could be related to the "display over other apps" permission found in settings > apps > special app access, and possibly "picture in picture" setting found on the "app info" page.
You should also check your GP settings and app update settings inside GP as well as notification settings for GP.
Preventing background activity for an app can be found in the battery usage setting of the app.