Go to list of activities, hold UP to open options, select "Remove from Favorites". A new screen appears with "Favorites" as the heading and the selected activity in the center of the screen. When I press any button, the firmware crashes and restarts. After restarting, the respective activity is still in favorites.
Using the Connect app is the currently only way to remove activities from favorites.
Forerunner 255S Music, software version 12.23