I tried several encounters with multiple enemies. Melee doesn't trigger it. Grenade doesn't trigger it at all (since you changed it today) except a few cases. Only super does it consistently. Solar weapons trigger it consistently too. I think you broke something, Bungie. Edit: Also, the buff seems to remove only get created when a solar weapon/ability is "equipped in hands" during the kill. Aside of that, swapping weapons completely removes the stacks.