QML Loader tab Navigate exception

Question

press 'Tab' key, first time, navigate successfully between the four controls. but since the second time, button1 can't focus.

first time, navigation chain:

checkbox1 -> checkbox2 -> button1 -> button2

since the second time, navigation chain:

checkbox1 -> checkbox2 -> button2

why?

//////////////////////////////// code: // main.qml

import QtQuick 2.5
import QtQuick.Controls 1.4
import QtQuick.Dialogs 1.2

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Item {
        anchors.fill: parent
        Column {
            CheckBox {
                id: checkbox1
                text: "checkbox1"
                focus: true
                KeyNavigation.tab: checkbox2
            }
            CheckBox {
                id: checkbox2
                text: "checkbox2"
                focus: true
                KeyNavigation.tab: loadItem
            }
            Loader {
                id: loadItem
                width: 100
                height: 40
                source: "OtherItem.qml"
                focus: true
                KeyNavigation.tab: checkbox1
            }
        }
    }
}

// OtherItem.qml

import QtQuick 2.5
import QtQuick.Controls 1.4
import QtQuick.Dialogs 1.2

Item {
    anchors.fill: parent
    Row {
        Button {
            id: button1
            text: "button1"
            focus: true
            KeyNavigation.tab:  button2
            Keys.onReturnPressed: {
                console.log("button1 enter");
            }
        }
        Button {
            id: button2
            text: "button2"
            focus: true

            Keys.onReturnPressed: {
                console.log("button2 enter");
            }
        }
    }
}