From our migration guide: https://docs.flutter.dev/release/breaking-changes/uiscenedelegate#if-your-app-supports-multiple-scenes

If you manually register a FlutterEngine with a scene, you must also unregister it if the view created by the FlutterEngine changes scenes.

// If using FlutterSceneDelegate:
self.unregisterSceneLifeCycle(with: flutterEngine)

// If using FlutterSceneLifeCycleProvider:
sceneLifeCycleDelegate.unregisterSceneLifeCycle(with: flutterEngine)

Issues:

1. It's unclear where to call unregister in the doc (so we should polish the doc)
2. This API can be error prone - developers could easily forget to call register/unregister (especially unregister).