When you add an external layout you need and must select a scene to write the events related so if later you copy or clone the scene the external will be linked to that copy too. Usually just use one scene and link the externals to that scene by linking.
For instance MainScene has an UI layer then create an external layout with the same name UI and create UIEvents as external Events. Then in MainScene add Link to UIEvents, this way if you need UIEvents to run in other scene that is not MainScene just add a link to that new Scene and it will work the same.
Hope this helps.