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Favorite FAB icon disappearing
You can replicate this by opening any app. Now click on the Floating Action Button with the star. It should be working as intended. Now scroll down, until the Collapsing Toolbar Layout hides the FAB. Now scroll to the top again. Press the favorite FAB again. Its icon disappears. It reappears when scrolling down and up again, or when recreating the activity by rotating the screen.