When I did some testing here is what i eventually found:
| User decision | message_id | was message received |
|---|---|---|
| User has approved notifications for app | 1714438464855660605 | yes |
| User has declined notifications for app | 1714438464855660605 | no |
| User never saw notification permission dialog, he has yet to decide | 0:1593443824341654%c4860296c4860296 | no |
The third case is interesting. Your app probably sent the notification id code to backend, it can be fetched even without getting permission from the user. But the app never asked the user for permission to send notifications. This way the backend is sending notifications and it looks like it should work but it does not.