Motion 5 rigging problem

I find that if I include a parameter (in my case it's an FxPlug filter parameter) in a rig, but then I delete that rig and save my project, the parameter still shows the joystick icon and seems to think it is attached to a rig. The option of adding that parameter to a rig is now gone, and I don't know any way of resetting it except for deleting the filter instance and applying a new filter instance (which causes me to have to lose a lot of work I've already done in that filter instance). Does anyone know if this is a known bug and if there's a "good" workaround once this has happened ?