This is a little strange but has happened to me twice- I have besieged a settlement with a sizable stack inside, and another one of that factions armies has come to the rescue and engaged me in battle. I play the battle and the reinforcements (the guys inside the city) are delayed, which means they never appear on the battlemap. I comprehensively defeat the relief force ('cos I'm so damn good) and put them to rout. The battle results in a victory for me, but the original army inside the city suffers no losses as they did not participate in the battle even though they appeared as part of the enemy's total forces. When the battle result screen is closed I go straight to the settlement captured screen with the option to occupy, enslave or massacre. The original stack has completely dissapeared- does not appear outside the city or anywhere. I never even assaulted the city- just responded to an attack by a relieving force.
That doesn't seem quite right to me- the forces inside the city shouldn't automatically surrender when the relief force is defeated. To me this seems like a bug- the inside army is counted as defeated along with the relief force. It does make things much easier for me though.