All of my Favorites were lost when I updated from v4.8.5 to v4.9.1 (I skipped over v4.9.0).
Here is what I saw:
I was unable to get them back by restoring my backed up preferences.
Fortunately, I use an old version of iTunes and had a backup of the v4.8.5 .ipa and was able to uninstall GoodTask v4.9.1 and re-install GoodTask v4.8.5, restore my preference, and get back to where everything was working.
But something is clearly not working in v4.9.1 with regard to Favorites.
If it matters, I am running iOS 13 but have NOT upgraded the Reminders database.