Bug: Culling Strike support gem doesn't work on my bow

I've got a Culling Strike support gem socketed into my bow and it simply NEVER works. I know the support gem only affects the bow attack, and that the gem is only for Rare and Unique mobs, but I keep testing those mobs types ONLY attacking using my bow M1 skill from slightly above the thresholds all the way until the rare/uniqie mobs are dead from normal attacks. Mobs just die from regular bow attacks, but NEVER works to trigger the culling strike.

Same situation here - on pathfidner. I just wasted 3rd lvl support gem that i got by trade :(

+1

I'm pretty sure it used to work early on, I remember using it in the campaign.

But now it doesn't work on any boss (or rare monster).