I don't know if this is intentional but a friend of mine discovered that while using 4-piece D3-FNC there are some discrepancies. For example like you said when the shield is out you have your SMG equipped, unable to switch to another weapon even when in cover. But if you have another type of weapon equipped when activating the shield it automatically draws your SMG except when you sprint or enter cover your character will draw the weapon they had before activating the shield.