I want to clarify that after posting this, I loaded my game back up, I make a manual save before starting a siege so I successfully reloaded my save right before besieging Seonon again.
I was able to perfectly replicate the above summary. I hit the siege button. I begin making catapults, half way thru making and putting catapults into reserve an ai lord from Sturgia with an army takes over the siege so I can no longer initiate an attack, but I still have control over making and sending out catapults from reserve. I send out all the siege equipment and destroy the enemy catapults and walls. The ai just never chooses to initiate the actual siege/attack to take over the town
So I leave the siege, then try to rejoin the siege to help the attack, which gives me the option to attack, but when I hit attack it instantly full crashes the game back to my xbox dashboard.
So I'm physically not able to siege a town, as a mercenary at least afaik, if an ai lord takes over my siege, then I can't even participate I just have to leave because it causes a crash trying to get the game to actually start the town attack rather than sit there sieging for the next 10 years.