it's your vassals that are causing that.
AIs do not always have a worst enemy. if they are cautious or higher with everyone, they have no worst enemy which is wonderful, you can trade safely yay!
but ... their attitude towards you, or anyone, is their TEAM's attitude towards your entire TEAM. PAs, vassals, and Teams that you setup in game options all count for this. because you have vassals, he's not pleased with your team ... he's Furious or something with Isabella and/or whoever that is researching Constitution, and that combined with his Pleased towards you is enough to put your team as his worst enemy.
same thing could happen if he had a vassal. same thing can mess up relationships between you and your own vassal, if you have more than one vassal. you can have two vassals that are Friendly with you, but if they aren't Friendly with each other, they're not officially at Friendly with you.