Melta rifle ammo is consumed by gun strikes when using the Inferno Pistol.  This only occurs when using this combination of weapons.
Perhaps it has something to do with the gun strike animation when using the inferno pistol, instead of a typical bolter animation/shot it looks like a melta/inferno pistol shot which is neat but also a bit jarring since I'm used to the former even when using a neo-volkite or plasma pistol so this change is interesting. 

Also, the inferno pistol talent Motivated Melee that increase melee damage by 20% does is not indicated as active through an icon under our health bar, is it supposed to? It would be nice to see this. Also is this perk only active when the inferno pistol is actively equipped or is the perk active when either weapon is equipped once the inferno pistol is empty?

I would like to add that the inferno pistol gun-strike to Zoanthrope has a bug or visual bug.

After initiating the gun strike, the character unholsters weapon but the attack animation is skipped, there is a animation frame jump and the weapon is holstered.

-> i don't believe the attack was applied to the Zoanthrope or at least not visually seen as being applied.