Custom notification sound in background(Firebase Android)

I use Firebase for notification in my Android application and try create custom sound for notification in background. Client app doesn't handle notifications, only android system. Notifications are sent from backend. Json structure:

{
    "message": {
        "token": "e1IMKJe...",
        "notification": {
            "title": "Test",
            "body": "Test message from server"
        },
        "android": {
            "notification": {
                "sound": "sound_base_notification",
                "channel_id":"test_chanel_id"
           }
        }
    }
}

In method onMessageReceived, I obtained response like this:

Bundle[{google.delivered_priority=normal, google.sent_time=1596444752253, gcm.notification.android_channel_id=chanel_id_test, 
google.ttl=23746652, 
google.original_priority=normal,
 gcm.notification.e=1, 
gcm.notification.sound=default, 
gcm.notification.title=Test, 
from=763552563,
 gcm.notification.sound2=sound_base_notification, 
google.message_id=0:1596...,
 gcm.notification.body=Test message from server, 
google.c.a.e=1, 
google.c.sender.id=29852264618, 
collapse_key=com.test}]

Could you help me understand where does the gcm.notification.sound2 come from although in json push exsist onle single key sound?

Note: Was testing on Android 10