Pre-patch i also definitely noticed that both you and the AI could maintain sieges (and naval blockades) that were started before the nonagression mission was handed out. As long as you didnt actually assault the castle you were ok.

Post patch i havent had a chance to check, but if its still around I think its gotta be a bug - maintaining a siege or blockade is definitely an act of agression whetever way you slice it. I'm guessing that since nothing is actually changing if you or the AI maintain the siege, there is nothing that triggers the failed mission event.

I wonder if maintaining a siege long enough that the city surrenders would trigger a failed mission - havent had that happen often on H or VH.