Grants X Life per Enemy Hit (Crossbow)

I recently acquired a couple Crossbows with Grants X (4, 5) Life per Enemy Hit on my Monk (Invoker). I am using Rapid Shot with a .05 attack speed and have 99% Pierce. I expect my 2,200 health pool to soar when I get chunked for 50% health on T15s. It does not. As far as I can tell, Grant X Life does not work at all.

I also have Thief's Torment, with Gain 25 Life per Enemy Hit with Attacks and it works as expected.

Is this a known issue? Do I misunderstand how Grant X Life works?