Qt.rect objects from C++ properties are readonly

import QtQuick 2.7
import QtQuick.Window 2.0

Window {
    visible: true
    width: 640
    height: 680
    title: qsTr("qtbug64115")

    Item {
        id: root
        Component.onCompleted: {
            console.log("=====PART 1=====")
            var rect = Qt.rect(50, 50, 100, 100)
            console.log(rect.x) //outputs 50
            rect.x += 500
            console.log(rect.x) //outputs 550

            console.log("=====PART 2=====")

            var rect2 = root.childrenRect
            console.log(rect2.x) //outputs 50
            rect2.x += 500
            console.log(rect2.x) //BUG!!! Should be 550, output is still 50
        }
        Item {
            x: 50
            width: 100
            y: 50
            height: 100
        }
    }
}