I'm playing as Volga Bulgaria. Starting with two settlements and being able to grab a third before their invasion, I have my generals defending because it is impossible to fight them in the field, or even in fortification stance, as they always travel in double to tripple-reinforcement range. I'm ready for a siege battle, but they will just simultaneously siege two of my cities with three stacks each. They have siege equipment, they have many catapults. But they do not attack. According to auto-resolve, they have every advantage imaginable, but just won't attack. I'm just on normal difficulty. This way I have to watch them for ten rounds starving me to defeat.

I've played Volga Bulgaria campaigns some years ago and that was never an issue.
Also, I think the fortification update makes the forts mostly useless. With, what, six? wide entrances there is as much gap as walls :D I get that vanilla forts were OP, but now they're plain useless. At least if you don't have a full stack or if you're defending against multiple Mongol armies.