Haven’t tried reproducing it so I might be full of it…
Playing waystalker on the Bile troll mine map. Have the “headshots-refund-cost-of-arrow talent,” and a chaos spawn was chasing me around. I ulted it in the face a few times and it didn’t do any damage. Works on other things, just in this one instance it didn’t lower the displayed hp of the target, didn’t seem to effect hp otherwise as well.
Oh this was on the beta.