Basically, HT_ProvinceControl trigger appears to be broken.
Mostly posting this a reminder to myself, I also keep a buglist of my own should AGEOD go through with the source code release, but feel free to adjust this for relevant events (by commenting it out/changing to something that works).
In the particular event in question, this leads to a situation where Persia collapses a few days after declaration of war on the Arabs
Rather anti-climactic for the Yellow Player...