How to diagnose notifications that have a GCM outcome of 'Success' but don't reach the device

This is really a continuation of another question I posted, except now I'm using Azure's push notification telemetry and Google discontinued FCM diagnostics in the Play Console.

I get reports from users saying they haven't received notifications. But the Azure telemetry shows results like these, even for these notifications:

"GcmOutcomeCounts": {
    "Outcome": {
        "Name": "Success",
        "Count": "1"
    }
}

Crashlytics shows no crashes, so the only thing I've come up with is putting calls to a logging API in to see where it fails or stop working, but it never even reaches the app in this case.

There has to be a way of figuring out exactly where and why the notification fails. What is it?

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