This has already been reported in forums against forerunner but I noticed this on my Fenix 5 plus yesterday.
I have the flashlight assigned to a long press on the down button. If you press and hold it then it is at full brightness but as soon as you release the button it goes to half brightness making it useless. Invoking the button from the controls menu works correctly at full brightness and I know for a fact it used to work correctly on my old Fenix 5 as it was a useful feature.