Yeah, this is really annoying.
This needs some fixs, as for the mercenary contract, where u should, in my opinion, have a fixed wage, but then you would be forced to join armies that you would be called in.
About that, the total autonomy u get and the unability of the AI to call you to join him into an army might disturb a lot his plans. The frame work is bad for that, that would be better that the AI lets u a time to join him, regarding of the distance u are when he calls u, and then if u don't join in a certain amount of time, u would lose relationship, lose wages as mercenary, lose influence and relationship as vassal, etc.
Also, adding a diplomacy aspect where the other vassals of ur kingdom could turn against u or, why not, if they are enough to think that, bannishyou from the kingdom, that would be cool.