Maybe it is too late to make it into the final update, but I just now observed this. Like the Title says, Arc Soul kills don't trigger Explosive Pact ("This weapon gains bonus stability and reload speed when activating a grenade ability. Healing grenades and grenade final blows grant additional stacks of the bonus."). That the first part "... when activating a grenade ability." doesn't trigger Arc Soul creation (Rift cast) is expected, but since Warlock constructs (including Arc Souls) should count as grenades for stats and perks ([url=https://www.bungie.net/7/en/News/Article/destiny_update_9_1_5]Update 9.1.5[/url]) it's kills should activate Explosive Pact because of "... grenade final blows grant additional stacks ..." in the second sentence. I also tested Hellion and Child of the Old Gods with the same observation. (Video only shows the behavior for Arc Soul.)