Mana leech is bugged?

I just got the "Ravenous Doubts" node on my ascendancy (mana leech is instant) and it is 100% not working correctly.

I'm using whirling assault, I have 6% mana leech currently and the skill says it is dealing between 150ish - 300ish main hand physical damage.

Obviously spinning that in a pack of monsters should instantly top off my mana bar but it is not doing anything.

Have the same feeling here, doesn't know if it proc on number of hits of the skill or "once" per skill used. Maybe enemies have a very very high leech resistance. Trying on a lv62 zone.