1. Board the Offroad in one of the front FFV positions while playing as the 'Rifleman AT' unit
2. Turn out so that the unit is in the standing FFV position that faces forward
3. Press the 'Switch to Primary Weapon' and 'Switch to Handgun' keys alternately to see that you can switch between these two weapon slots without issues (might need to check your keybindings)
4. Press your 'Switch to Secondary Weapon' while holding either your primary weapon or handgun and see that you do not switch to the AT launcher the unit carries on his back.
5. Use the 'Switch gun / launcher' keybind instead, and see that the unit does now shoulder his launcher.
Additional step:
Dismount from the Offroad and try alternating the 'Switch to Secondary Weapon' and 'Switch to Primary Weapon' keys to show that it does work normally, and the controls are only inconsistent in the FFV seat.