I'm sure a lot of people have encountered this issue, basically in PvE ammo never seems to drop for the weapon you are using, it's like the Juggler perk from Destiny 1 is always on. I'm not sure if this issue is platform specific of not, but the only thing that I know for sure, is that it get's annoying. Even the yellow bar enemies don't drop Heavy Ammo sometimes even though it was mentioned in one of the recent TWABs that yellow bar enemies would be guaranteed to drop Heavy Ammo. Please fix this or look in to this issue when you get the chance.