I think it was from Plague yeah. Also had a game where Plague was spawning 2 each time Infected Zombie got the last hit but havn't been able to reproduce it.
its only from plague (plagued by infected zombie out of corpse pile) not the swarms. it has been this way for as long as i can remember (year/s) why is this not wai, i like it. the few plagued champions created with death nova do not imbalance the game. both swarm and plagued used to have upgrades in the past and zombies were better for it. rotwrapper got this nerfed.
They fixed it after the bug first appeared about a year ago, at least i remember Sok saying they did. This new client seems to have unearthed a ton of old bugs.