Frozen Wardens take no dmg?

I’ve frozen a Warden with a cryo cannon and despite numerous shots to his head after being turned into an ice cube, he takes no damage. Is there something wrong with the hit box when he is frozen? A matriarch can be frozen and still take dmg when shot in the back. Yes a JD with a lancer GL seems to be the best method of disposal but what about the rest of us?