Strange QQuickView behavior

For Qml Online project we need to update QML preview window on QML file update in real time. However QQuickView works a little bit strange: it allows to modify window content without window recreation and works very fast. But if QML root element is an
Item {
...
}
after qxView->setContent() call I see previous QML file content with the new one as well. If root item is:
Rectangle {
...
}
all works just fine - I always see only recent content on QML file change.