Inconsistent behavior of Graph Api when on contact get by id

Question

If i query Graph Api with ID of contact that doesn't exist in my Outlook contacts I am getting Error 404:

{
    "error": {
        "code": "ErrorNonExistentMailbox",
        "message": "No mailbox with such guid."
    }
}

But if I use ID of the contact that exists in my organization (one of my colleagues Outlook contact's ID) I am getting 503 - Service Unavailable Error :

That behavior is inconsistent and could cause errors when trying to sync contacts using Graph Api.