Changing Loader source breaks references on QML objects: 'ReferenceError: ... is not defined'

I found a regression in Qt 5.10 when using Loader.

I have an app with main window, containing some properties and a loader. Loader loads few app's screens.

ApplicationWindow {
    id: mainwindow
    visible: true
    title: qsTr("Loader test")

    property string backgroundSource : "qrc:/background1.png"

    Loader {
        id: loader
        anchors.fill: parent
        source: "qrc:/screen1.qml"
    }
}

There is some code which I have to execute after loader changed its source. In this code I use properties of the main window. For example:

        MouseArea {
            id: area
            anchors.fill: parent
            onClicked: {
                console.log('Button on screen 1 pressed')
                console.log('Setting loader source')
                loader.setSource('qrc:/screen2.qml')
                console.log('Changing background')
                mainwindow.backgroundSource = 'qrc:/background2.png'
                console.log('Everything is ok')
            }
        }

Everything was ok up to Qt 5.10.0:

D libloader_test.so: qrc:/screen1.qml:31 (onClicked): qml: Button on screen 1 pressed
D libloader_test.so: qrc:/screen1.qml:32 (onClicked): qml: Setting loader source
D libloader_test.so: qrc:/screen1.qml:34 (onClicked): qml: Changing background
D libloader_test.so: qrc:/screen1.qml:36 (onClicked): qml: Everything is ok

But since Qt 5.10.1 I have this error:

D libloader_test.so: qrc:/screen1.qml:31 (onClicked): qml: Button on screen 1 pressed
D libloader_test.so: qrc:/screen1.qml:32 (onClicked): qml: Setting loader source
D libloader_test.so: qrc:/screen1.qml:34 (onClicked): qml: Changing background
W libloader_test.so: qrc:/screen1.qml:35 ((null)): qrc:/screen1.qml:35: ReferenceError: mainwindow is not defined

Attached small example with code to reproduce bug