This would be a Code bug.
It is always reproduceable. The steps that I use to do it are:
0: Go into training area, equip Larcin E2 (grand finale)
1: Shoot all bullets
2: Quickly switch to gadget before you throw the weapon (this "stores" the throw)
3: Use the expertise
4: While you're invisible switch back to your weapon (before you go into cover) and it throws the gun projectile and keeps you invisible
If you throw the weapon before switching to your gadget it will not work. If you go into cover before switching back to your weapon then it will just reload regularly and not throw.
It is always reproduceable. The steps that I use to do it are:
0: Go into training area, equip Larcin E2 (grand finale)
1: Shoot all bullets
2: Quickly switch to gadget before you throw the weapon (this "stores" the throw)
3: Use the expertise
4: While you're invisible switch back to your weapon (before you go into cover) and it throws the gun projectile and keeps you invisible
If you throw the weapon before switching to your gadget it will not work. If you go into cover before switching back to your weapon then it will just reload regularly and not throw.