Trapped Relievers' Castle Siege Length Bug

In certain situations, a large garrison can hold out indefinitely. Message will be something like "this castle will not fall without direct assault." The times I have seen this are when there are few defenders on the opening turn of the siege, then a large relieving force attacks the following turn and fails to break the siege. They end up trapped in the castle as well. But the game fails to recalculate siege length, so 300 or 400 men can hold out forever. You either have to do a horrendously costly assault, or abandon the province then re-attack, or bribe them.