With experimental build 0.37.115791 playing hardcore, the weapons flashlights (pistol and M4) will not activate once they are attached to the weapon. When you attempt to turn on, you see "turn on" as an option, but not "turn off".
If you separate/remove the flashlight attachment from the weapon and place it in your inventory, you can then turn it on or then off. However, there is no light projected in either case. The head mounted light/torch and the hand held flashlight work fine.