This is a very rare error and appears when you are using two monitors.
I have been trying for a very long time to understand what's wrong, and why the game behaves so strangely, and I have an assumption that this is happening because the Unreal Engine is launched in windowed-full screen mode by default.
This is why the game is launched in a window for the first time. This is done on purpose so that the user sets the full screen through the settings on their own.
Also the game goes into windowed-full screen when you press Alt+Enter.

So if you have 2 monitors and note that the weapon does not always shoot. Just go in Options and switch window/full screen mode.