If weapons are not fully loaded and trying to buy extra ammo for assets with multiple ammo consuming weapon types, the quickbuy will always purchase ammo for the topmost weapon type, regardless which weapon was selected. This is extremely aggravating with assets such as Vulture A, Rifleman A and Thanatos D, which have the secondary weapons on top, causing accidental MG or SRM ammo purchases.
Here's how everyone can try it out and see that the issue really exists:
1) get an asset with multiple ammo using weapon types and free slots for extra ammo, such as the ones above
2) fire off a couple of rounds from all the weapons
3) select the weapon you want to buy ammo for (not the topmost weapon type)
4) repeatedly tap the quickbuy button
5) now you should have at least half of the free tons filled by the topmost ammo type, instead of the one you selected
Note that the problem does not occur when purchasing extra ammo when weapons are fully loaded.