This problem occurs on Edge devices when inside an App Menu. If you press the Start Button, it will start a native looking activity. When you stop and exit the native activity, you will also be exited out of the App.
You can observe the problem easily with the MenuTest Sample App on an Edge Device. Start the App. Press the Menu button. Press start. It will start a native activity even though there is not activity code in the app.
It happens on both the Edge 1000 and Edge 520. I don't have an 820 to test. I don't think this can be an issue on any other device because the start button and enter buttons are not separate buttons like on the Edges.
The App Menu Delegate doesn't have an onKey function so it's not possible to handle the Start Button press in the App. I'm not seeing any way to prevent the unexpected behavior. It seems like the simplest solution would be to ignore the start button in an App Menu but this would be a change on the Garmin side of things.