{Gameplay} Devouring the undead

I was in a game and the tahm kench of our game tried to eat the passive revived Sion, and it didnt work. Tried it again later and same thing happened. Can understand this not happening with Kogmaw, but I feel it should happen with the undead sion. Thought it was worth looking into, havent yet tried to replicate with the 5.14 patch