I have never seen a vent bushed before. It may be that the bushing, or the vent liner itself is extending into the bore, and is hooking your jag. The only way to know is to remove the liner, and put something in to measure the distance exactly from the outside of the barrel to the bore, and then measure the threaded portion of the liner. I would run the RR down with a jag on it, with the gun cleaned, and empty, to see if the jag hooks on the edge of that bushing, too, when the liner is out of the bushing.
You may have to remove that bushing, and reduce the length of it to keep it from hanging up, or hooking that jag. Otherwise, you simply will not be able to clean all the way to the bottom of the breech, and the gun will begin to fail to fire at some point, when the crud build up and blocks part or all of that TH.