It's possible that it only works that way on console, but I think I've heard it confirmed that the mana drain and stun spillover on PC/Mobile, too. In my mind, it's a flaw in design, but you can use it to your advantage.
I sometimes feel a little bad about using a single cast to kill two troops in PVP and then stun and drain a 3rd, unless its a Bonedragon that someone is trying to protect by putting it in the 3rd slot.
Wulfgarok is another one that has some funny interactions with resurrection. If I remember correctly, I once killed a Dragon Soul with Wulfy, had it resurrect in first slot, had Wulfy devour the new one and then had it resurrect again. I was like, WTF just happened?