Ah interesting, okay no problem, I know what happens. Its not your fault.
So to disable the usual work of Diplomacy I set aggression to the max before attacking monsters. Now when you haven't visited the monsters it might be possible that it shows that they want to join you because their aggression level is still unchanged.
What I have to do is to run a function at map start to set all monsters to aggressive, then the vision spell will not lie to your face
Good catch