Ob Nixilis' 2nd Ability Doesn't Kill Undergrowth Champion

Hi! I recently played a game in an event where I tried to kill an Undergrowth Champion with Ob Nixilis' 2nd Ability. This simply knocked its power and toughness down by one and then the Undergrowth Champ killed my entire board of attackers.

My expectation was that Undergrowth Champion would die since it was Nerfed in a previous update. If this is a bug, please fix it. Thanks!