This fireteam of 2, had the other player already done the mission? I did the unlock prep yesterday, then my friend came online so we did it again for him. All wizards, no issues. Beat mossion all good. Then later I did matchmade Overthrow with low level randoms, wizard spawned and I led them to it, they probably had no idea why. Today I did 2 overthrows solo, no Wizards either time, so I guess once you've unlocked it you don't personally get the wizards again. So my guess, and I'm probably wrong, is your friend already unlocked the mission and was fireteam leader, maybe stopping you from getting them? Just a thought, might help. Good luck.