Lucario Avenging Knuckle

Lately my opponents have used midnight shuriken, to knockout my benched fighting Pokémon and my random attributed Pokémon for me to not receive lucario boosted attacked. Correct me if I’m wrong but the literature on the avenging knuckle states “any fighting Pokémon knocked out by damage” , so with those two knockouts by midnight shuriken it’s seems it goes based off the last Pokémon that was knocked out which in this case was not a fighting type. Is this a bug or legit play?