Putting Repeater and standalone item in GridLayout leads to weird behavior

[Stefan Monov76]

My code:

import QtQuick 2.6
import QtQuick.Window 2.2
import QtQuick.Layouts 1.3

Window {
    visible: true
    width: 640
    height: 480

    GridLayout {
        id: grid
        anchors.fill: parent
        columns: 6
        Repeater {
            model: 7
            Rectangle {
                color: "blue"
                Layout.fillWidth: true
                Layout.fillHeight: true
            }
        }
        //Rectangle {color: "red"; width: 20; height: 20}
    }
}

It shows this:

Which is fine.

But when you uncomment the line containing color: "red", the layout breaks:

Any idea why that is and how to fix it?