Hey GutoxAngel,
so I encountered the exact same bug and could replicate your being able to shoot with other button inputs instead of MLB+MRB.
The only fix I found working for me to be able to use the two mouse buttons again in Archwing controls is to reset the key binds to default in the settings menu. Afterwards, setting them back to my previous configuration still allowed me to use the archgun normally.
So I suggest resetting to default, but before doing so it might be a great idea to take screenshots of your button configurations.
Cheers. o/
