Won as Malz Support, didn't complete Arcade Boss Mission.
I won a game (1st time malz support) and didn't complete the Battle Boss! Malzahar mission. Best I can figure is I also got a chest (got an S, he's super easy to play) and that interfered with it somehow. But either way I have a screenshot of it if necessary. I finished ziggs earlier if that has anything to do with it.