Mixing content-available and alert

FailingSilentNotificationWithAlert.md

{
    "aps": {
        "alert": {
            "body": "Test message",
            "title": "Optional title",
            "subtitle": "Optional subtitle"
        },
        "content-available": 1
    }
}

Silent notifications tell an application to load new content and update the user interface in the background. Alerts, badges, and sounds however are not delivered in the background and are visible to the user. A notification can be visible to a user or it can be silent. It can not be both. This notification contains both an alert and the content-available key that marks this as a silent notification. iOS will not be able to reason about how this notification should be delivered and may fail to deliver it completely.

You can validate your push notification payloads to find problems like this one using the push validation tool