AI and landgrabbing

We, as players, look at proximity, rivers, cultures, historical borders, aggressive expansion, diplomatic costs, etc. However, the AI simply decides, "I wanna!"

To elaborate: When I call France as my ally in MY war, it conquers the Dutch region (Harderwijk, for instance), peaces out and takes that land, even though I am between the county (Harderwijk) and the main country (France). My relation is +200, and I professed my trust in them whenever possible, basically. You have the option to buy it from them... doesn´t work.

The last war involved the Friesland area (northern part of the Netherlands IRL), and it wanted to do the same. I had to reload because this was getting ridiculous! This clearly is designed by choice, and no way in Ragnarok is this a coincidence. It´s already frustrating when vassals peace out for us, but now we have to speedrun to capture a province; otherwise, our ALLIES get it for themselves and peace out?! In EU4, allies were already unreliable, but this is worse.