When you assist a siege defense, after the battle ends you get prisoners, but all parties in your army do not. All of their prisoners are deposited into the fief you helped. This happens after battle, you don't even step into the fief,
How to recreate: Wait for a siege against your kingdom. assist the defenders. After the battle look at your armies parties and they have no prisoners, they all go to the fief.
How to recreate: Wait for a siege against your kingdom. assist the defenders. After the battle look at your armies parties and they have no prisoners, they all go to the fief.