Magic Duel not Magic Duelling?

[Cohen]

I know there's a Ryleh player that is assassinating my guy with his Starchild.
I've sages and I scripted the astral sages to Magic Duel the enemy guy who should be an S1 mage. (since the assassins were well equipped with Bottle of Living Water and other equipment, it was fine to accept the risk of 50%).
After first success, the next time the assassin was S2 due to starshine skullcap. My Sage refused to cast Magic Duel (!!!) casting instead a Communion Slave (dunno for what master...) and died.
I know he had more chances to die than to survive, S1 against S2, in single magic duel, but he had no chances to survive against a water elemental + the assassin. It's the common bug of AI "thinking" on its own? Or it's something else?