I've noticed harpies invulnerable to several similar things! Killing with my 'Devil You Know' should increase damage against other enemies but it doesn't :/
English
-
Maybe the problem lies in the harpie itself, interesting. Definitely doesn't seem intentional then. Even shanks trigger the reload, it would seem a harpie would. They also don't explode with appropriate perks now that I think about it; golden gun's explosion perk for example.
-
I tested it yesterday on the heroic daily story. Killing a normal Vex and then hitting the Harpy dealt 1000 damage with hand cannon critical. Killing a harpy and then hitting another harpy dealt only 800 ish. I don't think it's intentional. Maybe they'll fix it soon.