It's a little more troublesome in the raid when you have to take down the bathers and heavy ammo just isn't dropping.

Heavy ammo should drop every time you kill one of them (that’s how it’s been for me anyway)

Sadly that has not been my experience.

That's odd because every time I kill a bather - literally - one or two bricks of heavy drops. Every. single. time. It won't necessarily work if you do not do damage or enough damage to them though and another guardian does. In the garden, the reason they have so many orange bar dudes, prior to beginning the dogs encounter, is to refill your heavy as well (at least in my experience as I can fill my heavy 3/4 to full for every run). It seems that Bungie has replaced synth with the numerous orange bar enemies and, of course, the yellow bar enemies as the drop rate for heavy from those enemy types is close to guaranteed (again, in my experience).