If this still happens, and you want to get rid of favouritesA/B once and for all, you can edit osu!.YourWindowsUsername.cfg file with notepad and delete all song codes after FavouritesA = and FavouritesB = . The file's start should look like this:
XPJ38 wrote:Just noticed that if you happen to delete FavouritesA and B, they come back after restarting the game.
TreeSortMode = Search
TreeSortMode2 = BPM
Don't forget to make a backup first though in case something goes wrong