If anyone's confused she basically used that mirror to make that "bastard" the creator even though he originally wasn't, because if you remember that mirror makes anything one believes to be real into reality, and since fairies are always so pure and honest he believed the mirror's illusion thus making him the creator the causality, in consequence his magic was sucked out because we also learned that fairies lose magic when they create an item (Iris said it herself), Now that black eyed fairy has both the causality and all her magic returned to her, since that guy literally got Thanos snapped causality wont try to return to it's "original owner" since bro is nonexistent