Properties of dynamic types not resolved correctly in grouped syntax

import QtQuick 2.0
ComplexButton {
    background.color: "#fff000"

    Component.onCompleted: {
        console.log("main.qml.onCompleted: " + background.color.toString());
    }
}

import QtQuick 2.0
Item {
    property alias background: backgroundImg

    // No error.
    //Rectangle {
    //    id: backgroundImg
    //    color: "red"
    //}

    // Error: main.qml:4:16: Cannot assign to non-existent property "color"
    Image {
        id: backgroundImg
        property color color: "red"
    }

    Component.onCompleted: console.log("ComplexButton.qml.onCompleted: " + backgroundImg.color.toString())
}