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This has something to do with your camera or the camera apps you have installed. If you start your flashlight from quick setting and then you start camera it will disable this quick setting option.
Please give a list of your installed applications or try to delete their cache.
By any chance does it work when you restart your device?
In my case, the flashlight icon disappears, and then I go to diagnostics and the test fails, if the phone is restarted the flashlight appears and passes the test...after a while the Flashlight icon disappears again.