FCM not returning Unregistered Device for uninstalled app

I'm trying to figure out some issues with my push notifications (iOS APNs) with Firebase FCM.

I tried uninstalling my app and send the unregistered token to FCM, but FCM is NOT returning "Unregistered Device" error message, and it returns success instead. This way I couldn't tell for sure if the tokens are still valid.

{
    "multicast_id": id,
    "success": 1,
    "failure": 0,
    "canonical_ids": 0,
    "results": [
        {
            "message_id": "0:id"
        }
    ]
}

How can we track if the tokens are still valid? Thanks!

Update:

When testing with the release build, after I uninstall, and reinstall the app, the device token gets invalidated. If I don't reinstall, it still says token is still valid. When testing with the debug/dev APNs, the token is always showing valid regardless if you reinstall.