Ok, so this happened to me once on accident, and I repeated it 2 more times in different situations to see what was going on.

So it has to do with an ally of yours, or even someone you are at peace with, having a castle under siege by a third party army that can be your enemy or not. You use a Mensuke to bribe that army to join you before the siege has started. The army jumps to your territory, but the siege continues at the castle of your ally or peaceful neighbor.

As far as I can tell, the AI can't do anything about the siege. All 3 times after the required number of seasons had gone by, I took ownership of that Castle. Twice it was my ally and they smiled at me like nothing had happened. I think if an army is bribed during a siege, there should be some code to tell that siege to break off, OR ask if you want to declare war on that nation when you attempt to bribe, since you will be continuing said siege.

I tried it different ways and every time the AI just stood as close to the unit as possible, that I had bribed, maybe trying to get to it to kill it, but it they would not enter my territory or declare war on me since I did not start the siege.

I am sure it is an easy fix, and it may have been mentioned already, but I didn't read every post and most of them just said BUG, so who knows.

Hassie