Android sometimes kills the realtime notification
The listener is sometimes killed but the notification is left. Other times the entire notification is killed. This is all due to RAM pressure and Android's notification lifecycle.
It might be possible to reregister the listener. It also might help to increase the importance of the notification. If the notification is dead, launching Privacy Cell restarts it. But, if the notification itself is not dead, it might be necessary to add a button to Privacy Cell to restart it to reregister the listener.