Mob Riding AI Overwrite

So a Mob's AI should be overwritten by the Mob's AI who's riding it right? But if I'm trying to get a hostile Mob on an Iron Golem (For Map-making - to get an "evil Golem") the Iron Golem starts attacking me (only sometimes though) like expekted, but he still kills the Zombie/Skeleton etc on his head (even if they are on the same Team).