Regarding new expansion. Question.

[H Gilmer3]

I'm besieging a fort of an independent tribe with my army of 113 power, defender at 31 power. It's taking a long time. While besieging said fort/city, I become allied with another faction. Allied faction moves army of 60 power into region I am besieging. Siege ends and he gets the region?

I had breached all walls and set my army to assault, no battle was fought and it is the allied AI region after the turn is over.

[Pocus]

That's a bit rude from them

Seriously, several provisions have been taken to avoid someone snatching a region, but the one besieging first is definitively not the one who will systematically get the region, because that too would be prone to exploiting.

The AI on the other hand will retrocede a region if you have more neighboring regions you own than it or if it is a client state.