Grob Twin Astir getting "stuck" in a slip
Have you tried it both directions? The rudder is hinged on one side on the Twin Astir, so it likely behaves differently one way versus the other. It has to do with stalling the vertical fin. If you push right rudder, the tail of the plane moves to the left. The relative wind tries to stay parallel to the centerline of the rudder, so it is coming at the fin from the left. As you increase rudder deflection to the right, you are able to increase the AOA on the vertical fin enough to stall the right side of the vertical fin, which will pull the rudder full to the right. I am guessing that you can do this with a slip with rudder into the hinged side, and not the other way around. Why? The airflow can stay attached on the non hinge side due to the gentle radius at the fin to rudder transition, but not on the hinge side due to the abrupt change in contour at the fin/rudder. Since the flow stays attached, the rudder has a bit more authority in one direction than the other, so you can generate more sideslip and get into what is often called "rudder lock". This is where, as you described, the rudder stays completely deflected one direction, and you have to push, maybe very firmly, to get it to come back to center. But once centered again, it behaves normally.
It is not unique to the Twin Astir, but seems to be more common on planes with a side hinged rudder.