Hi! Guendolin here from the dev team.
Thank you for your feedback and bug report. Unfortunately this is kind of working as its intended. Let me explain.
In the game we bind some actions to pressing a button and some to holding them. The bayonet action for the Lucios Lasguns is one of those cases. You hold the weapon special button to charge up a melee attack and release it to make the attack. This way we want you to be able to charge up the attack while moving towards the enemy and then release it as a bayonet charge!
Since you can’t hold the “mouse scroll up/down” inputs we cant do the attack. Force Staffs melee with the psyker works in a similar manar and wont do attacks either with weapon special bind to the “mouse scroll up/down”. You will find a similar issue with the plasmagun. When holding the weapon special button you will vent the overheat from the plasma gun, but you can’t hold “mouse scroll up” so that won’t do anything.
Another example is binding the primary action to the scroll wheel and you will notice that most melee weapons won’t work since most of the attacks start with you holding the primary attack button.
Its an unfortunate side effect of the nature of the mouse scroll input and how its behaves.
Thank you for your feedback and I hope this clears up how that input is meant to behave.
Best regards / Guendolin