Bug in valuetype value write-back semantics

import QtQuick 2.0

Item {
    property variant v1: Qt.vector3d(1,2,3)
    property variant v2: Qt.vector3d(4,5,6)
    Component.onCompleted: {
        v1.x = 8
        v2.x = 9
        console.log(v1 + ", " + v2)
        // expect: QVector3D(8, 2, 3), QVector3D(9, 5, 6)
        // result: QVector3D(1, 2, 3), QVector3D(4, 5, 6)
    }
}