QML Window declared inside an Item is not visible

According to the document of Window type in QML:

"A Window can be declared inside an Item or inside another Window;"

However, the following code does not work with Qt 5.9.5 , Qt 5.10.1 and Qt 5.11 Beta 4 on Linux/Mac. The Window will not be visible. 

import QtQuick 2.9
import QtQuick.Window 2.2

Item {
  Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")
  }
}

However, the code above works with Qt 5.9.4 , Qt 5.10.0 . The window is visible. 

This bug will affect multi-window applications,  make it difficult manage multi-window object instance.

An example project is attached to reproduce the bug.

Thanks