Cast on Critical Life Bug?

Hello, using the "Cast on Critical" Meta Gem causes damage to yourself when attacking something. It is especially noticable with high HP targets, Bosses etc.

This is whilst using Blood Mage Ascendancy AND Blood Magic. I am fully aware of the rather massive Life Skill costs of the casts themselves.
The damage I'm talking about is a Life loss once the target is actually hit.

Example: A briefly channelled Bonestorm with ~200k Dmg unleashed on a Boss completely deletes my HP pool, once the target is actually hit by the projectiles. No death though, not below 1.

This.. damaging behaviour.. immediately stops once I unslot Cast on Critical.
I've tested it both with Bone Cage and Comet, to see if the slotted Skill makes a difference.. it does not. Neither does removing any or all of the Support Gems.

Help, please. I like Crits..