This is true for the main game as well, but I play CK2+ mostly so I'm posting it here.
If and when you conquer the Pope as anything on a lower tier than Emperor, and if you don't vassalize him as an Emperor, he will consistently declare war on you every time his truce with you is up even though he is landless. He has no troops and if he hires mercenaries and you beat them you can only white peace him.
My proposal is that if you control the bishopric of Rome as an Emperor, you should be able to crown an antipope as the official pope, and if you're a king or lower, the Pope shouldn't be able to attack you unless he gets support from a landed character to do so.
If and when you conquer the Pope as anything on a lower tier than Emperor, and if you don't vassalize him as an Emperor, he will consistently declare war on you every time his truce with you is up even though he is landless. He has no troops and if he hires mercenaries and you beat them you can only white peace him.
My proposal is that if you control the bishopric of Rome as an Emperor, you should be able to crown an antipope as the official pope, and if you're a king or lower, the Pope shouldn't be able to attack you unless he gets support from a landed character to do so.