Loadout -> Copy From... copies the wrong heavy weapon.
In Loadout A enter vehicle mode.
Equip a ranged weapon i.e. Imperator Vandal
Equip a different heavy weapon i.e. Fluctus
In Loadout B select Copy From... and select Loadout A
Loadout B has the ranged weapon equiped as heavy weapon i.e. Imperator Vandal instead of Fluctus.
NOTE 1: This might not happen if you have only one archweapon with gravimag.
NOTE 2: I did some testing and for me it seems to be always setting Imperator Vandal as the heavy weapon. It was my first grvimag archweapon so instead of setting the ranged weapon it might be setting the first gravimag weapon instead.