I spotted this when I tried to retract a vassal that held a title that a regent had taken from me, and later given out. When I tried to retract the vassal, he rebelled, and when I beat him, he ended up in my prison. However, when I then tried to retract the vassal, he rebelled again, which I instantly won because he was still in my prison. I could not try to retract again, as in the interval, I was placed back under the Iron Throne, which has been (annoyingly) stuck on Autonomous Vassals for ages*, blocking the action. In vanilla, there is a +++++ modifier for prisoners to force them to give up titles or vassals without rebelling, which did not happen here.
*This has also blocked me from assigning troop leaders except during mega-wars, which is rather frustrating.
*This has also blocked me from assigning troop leaders except during mega-wars, which is rather frustrating.