I can't say I have seen this, either in BI or in RTW self.
I have seen one example of where the enemy forces were arrayed, but when charged, my forces were unable to strike theirs. Nor did theirs move. It was liek they were frozen.
Quitting and reloading fixed that but not at once. I had to move about a bit first, then try again. That next time we met I routed them from the field.
Does your bit return after reloading? Does that occur with only that town (maybe a loading bug) or at every siege?