[b]Edit:[/b] Bungie confirmed in the [url=https://www.bungie.net/en/Explore/Detail/News/47602]1/31/18 TWAB[/url] that this bug is known as part of 2.1.4. It is also known that the Sniper frame in question can potentially disappear from your inventory after logging out.
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Fallen are not dropping any Compound Ether to progress the Black Armory basic sniper rifle frame. Specifically, this is the frame you get as a drop from Ada-1 after unlocking the Gofannon forge on Nessus.
From reading some guides online, when you're on this step of the frame, any Fallen should drop Compound Ether on any kill, anywhere. It's also supposed to drop fairly often (more than half the time). I've tried killing Fallen on both EDZ and on Nessus, with headshots, with a Black Armory sniper rifle, etc etc, with zero Compound Ether dropped. After asking around on a couple Discords this definitely seems wrong.
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10/10/19 here in october and this issue still exists for some reason. I finished the first frame that introduces the forge activities and went for a hand cannon after that, and no compound ether or weapon core drops. though the quest didnt dissapear after relogging and I did try abandoning and re acquiring the quest, no dice.