If you attempt to set the Favorites tag on a Report that has been shared from another Workspace ... the Favorite Icon appears briefly ... but it does not remian visible.
When you then select Favorites .. you get the message "No favorites yet"
This is the case with Android Mobile. My understanding is tha tthis option works ok with iPhone Mobile (but I can't verify this)
Based on my test in the latest Power BI mobile app android version 112683, when we go to Shared with me section, open a report and click on Favorite button (Favorite changes to Unfavorite), then back to the home page and choose Favorites, we can see the favorite report listed there. Please update the Power BI mobile app to the same version as ours then test again.
Best Regards,Qiuyun Yu