YES, this happened on multiple fronts at same time too, so you would think you would for sure lose at least SOME of the castle's right? i started getting suspicious because i personally saw multiple sieges happening at same time that were supposed to be so unfair..only to lords apparently be like "nope, keep your stinky castles" i think they may starve for some reason....However if they never take cities sturgians can never stabilize as AI might think we are fine and keeps going against 4 factions at same time.
its worth the try, to mess with these settings.