Did not get credit for winning my round in Prize fight for mission

Won my round of prize fight. There was a Karthus on the enemy team - he killed me through his passive after I won the round but before round 3 started (where the enemy solo champion would have fought me). My solo champion lost the solo round.

Even thought the event said I won my round, I didn't get credit for the mission. I suspect it was due to Karthus killing me in his Passive before the solo champion loaded into the 3rd round.