Mage Questline Doesn't Reward

In a game - I was supposed to be rewarded the final thing in the Questline for Mage (Sorcerer’s Gambit). After that, I used the spell Evocation. The minion I was supposed to be rewarded with that has a battlecry of +3 spell damage the rest of the game disappeared from my hand. I should not be credited for the loss of the game I was in.