Heavy Weapon loadout is listed in the Vehicle Mode, but is tied to the Tenno loadout slot instead

[--Q--uixotic]

TL;DR - Heavy Weapon selection is saved on Tenno loadout, not Vehicle loadout, but is displayed on Vehicle loadout.

You can add Vehicle loadouts, but the Heavy Weapon selection is saved with each Tenno loadout, ignoring the Vehicle loadout selection.
This is very misleading and confusing.