Acutallyt his also goes for 'taking over sieges'. If you have an army in allied-controlled enemy province, and your ally settles´for peace with that enemy and the province control again goes to the enemy, you don't automatically start a new siege. You have to walk and the stop again, then your army is 'registered' by the AI and you can start the siege. - This is a problem for armies on islands as pointed out. You have to go by with a fleet to make that move.
It is as though, when a province changes controller ALL present armies are NOT considered, and you have to make the AI aware that it exists. This can also be abused. If several nations take over a province, and the controller settles for peace, then the present nations are NOT sieging. If someone the arrives with new troops THEY take over the siege regardless if they had presence previously. Not quite fair.
If this is a feature, I'd like to hear the justification.