This has been happening on and off recently, where the favourites album just disappears, and when I close and restart the app, it comes back, but not this time. I've tried restarting my phone multiple times, turning it off completely, leaving it for a while before starting it up again, cleared cache and data of media storage. The pictures in their own separate albums are still marked as 'favourite', but it's just that there is no 'Favourites' album at all.