Is leech broken?

I've been trying an acolyte monk build, and the mana leech is rather strange. I'm not sure if it's because of the extremely bad server lag, though. It never works properly; you either get nothing from hits or everything from hits. Why is it like this? I know that there is a leech-resistant, but this strange and random behaviour doesn't make sense. What have you done, GGG, with the leech?

everything is broken.