How to get messages sending errors through FCM?

Question

Android

I send a push to 6 devices. This response comes up:

{
    "multicast_id":8207570606454123569,
    "success":3,
    "failure":3,
    "canonical_ids":0,
    "results":[
        {
            "message_id":"0:15333543619395344b06c37bf9fd7ecd"
        },
        {
            "message_id":"0:1533354361940253%4b06c37bf9fd7ecd"
        }
    ]
}

The documentation says that results will contain message processing statuses. The response shows that success=3 and failure=3, while there are only two entries in results. How to find out these 3 errors (what kind of errors and what registration_id they are)?
In general, why is this so (only two records in results)? I expected that the results will contain statuses for all submitted registration_id (in my case there are 6 of them).