I don't know if this is a bug, but it feels like one.

I was doing Nexus Blitz when Prize Fight came up, everyone had died and it was just Wukong (my team) and Karthus (enemy team) left. Wukong killed Karthus, Karthus while in his passive tried to kill him, but failed, he used his ult, but Wukong still had hp, then they just stood there for a while until the ticking dot (from the ring of fire slowly killing you to help move things along) killed the last chunk of HP Wukong had, and the enemy team was declared the victors due to Karthus's passive still being up even though he died first, and it was the ring of fire that killed Wukong when we had already won.