I don't know if it is a new feature or individual card thing but AI will protect very valuable creature cards like lord type that give others bonus even if it could win the game by making the attack also with them.I noticed that 3 times in different games involving different cards.AI indeed protected lord type of cards always but not when it would make the attack to win.Unless the problem in my examples is that AI for some reason didn't calculate correctly the damage it would inflict.Of course this way AI gives an extra turn to human to get a card/s that will change the winner.

AI didn't attack the turn before in the screenshot you see.I can only block with Vampire Nighthawk as AI's creatures have all flying no matter what I block I will loose, still AI didn't make the attack as didn't want to loose Megantic Sliver.My assumption was correct as next turn that attacked for the win AI again didn't attack with Megantic Sliver.