Mana Leach working weird

I'm able to cast lightning arrow continuously with my mana flask active but as soon as I start attacking a boss at all I get my whole mana bar sapped away.

I've tried changing items out and moving passives around but it almost seems like my mana leach is working in reverse against map bosses specifically.

I do want to note that my tests go from firing at nothing and keeping my mana full as compared to firing at a boss

Any help would be appreciated