This happens In legendary difficulty?
i will try also.

Well, no option for CS for Parthia exept the diplomacy menu,but i believe from what ive seen during my previous campaign as Macedon, at least with patch 2, its like trying to get trade agreement, no chance at all.
And the funny thing was that there was a time i had 18 CS, some of them fighting with each other.

It will be interesting without the option for Parthia, trying to purely conquer and hold 140 settlements to win.