Why Are vassals of mine not diplomatically limited?

I started an iron man run as naples,my vassal Achaea, is able to start wars with my other vassals like morea albania, negroponte etc.. but then i ignored it but i lost my mind when it started its own wars with neutral countries around me, it wasnt a coalition or independence movement against achaea, it was an offensive war caused by achaea and i had an option to join or not, i obviously joined coz i didnt wanna lose a vassal or its land, and i became the war leader. idk why this vassal is able to start its own wars, like colonial subjects. Is this Normal or my run is bugged?