Attached properties are missing inside mjs modules

import QtQuick 2.12
import QtQuick.Window 2.12
//import './app.js' as App // works fine
import './app.mjs' as App // produces error

Window {
    visible: true
    width: 720
    height: 480
    title: qsTr("Keys")

    Rectangle {
        id: rect
        anchors.fill: parent
        focus: true
        Keys.onPressed: {
            console.log('qml.rect.Keys.onPressed', event.key)
        }
    }

    Component.onCompleted: {
        App.main(rect) // inconsistent behavior here
    }
}

export function main(rect) {
    console.log('mjs.rect.Keys', rect.Keys); // prints undefined
    console.log('mjs.rect.Component', rect.Component); // prints  undefined
    rect.Keys.pressed.connect((e) => { // throws TypeError
        console.log('mjs.rect.Keys.pressed', e.key);
    });
}

function main(rect) {
    console.log('js.rect.Keys', rect.Keys); // prints QQuickKeysAttached(...)
    console.log('js.rect.Component', rect.Component); // prints QQmlComponentAttached(...)
    rect.Keys.pressed.connect((e) => { // emit events
        console.log('js.rect.Keys.pressed', e.key);
    });
}