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My experience also fits with Vynd.

However I have noticed a serious nuisance. Suppose an enemy attacks a province containing a castle, where I have patrolling troops, maybe even just PD. I have reinforcements moving in to that province from outside. The reinforcements will never fight! Instead the battle will be between the enemy and the patrollers only, then the reinforcements wind up inside the castle.

This is extremely aggravating.

Is there any way to prevent this? If not I think it should be considered a bug.

But if there are no patrollers (everyone in the castle is set only to defend and there is no PD), the relief force only will fight the invaders. I think this part is correct.