I am implementing push notifications using Web Push for a web application running as a PWA on iPad. When sending a push notification to the web application, the push service returns an HTTP 200 response. However, no notification is displayed on the PWA, and the Service Worker's push event is not triggered. If I create a new push subscription and send the notification using the newly issued endpoint, notifications start working again. Notifications are not disabled in the iPad notification settings, so I would like to understand what could cause this situation. I am using a commonly used JavaScript Web Push library to send the push notifications. I would also like to know whether there is any way to detect or confirm that a push subscription is in this state, other than observing that notifications are no longer being delivered. In particular, if the push service returns HTTP 200 but the notification is not delivered and the Service Worker's push event is not triggered, is there any way for the application server or the PWA to determine that the existing push subscription is no longer usable and needs to be renewed?