This would be great.

yep.
It's not really a bug, its a feature so you don't run up to a dead guy in the middle of a battle, pick up his rifle and instantly gain all his ammunition, it doesn't work like that in real life.

I think it should stay how it is, with you either picking up the rifle and using the remaining ammunition in the magazine, and then picking up more ammo from the vest, or picking up the weapon + ammo after the fight.