In Arma, many backpacks come pre-packed with items, if you spawn the backpack, it automatically has the pre-packed items in it. You also cannot add items specifically to a unit's backpack, they fill the vest and uniform only. So, to match your unit's arsenal loadout 1:1 to the in-game result, the gear is all manually added on spawn.
Your units should have no problem reloading their weapon on spawn.
I can try to put a hack in to force all AI to reload empty weapons on spawn, but it should work automatically granted you provide them with valid ammo.