I don’t know if this is expected behaviour or a bug, but I lost all my favourite subs too, so you’re not the only one who has been affected.
I was only using the favourites feature because I wanted a way to list all the communities that I’m subscribed to. As far as I’m aware, there isn’t a way to view your subs list yet (though please correct me if I’m wrong).
It’s a mobile web app, so iOS users can visit https://wefwef.app using Safari, then click the Share icon, the click ‘Add to Home Screen’.
I don’t use Android so I don’t know if there’s a similar process to turn a link to a website into an icon on the Home Screen. I just did a web search and found this website, which I hope is useful for you https://beebom.com/ways-turn-any-website-android-app/
(Also I’m new to wefwef and this is my first comment using it)