[REG: 5.15 → 6.2] binding doesn't update when it should

import QtQuick 2.15
import QtQuick.Window 2.15

Window {
    id: root

    width: 640
    height: 480
    visible: true

    ListView {
        id: lv

        model: ListModel { }

        delegate: Item {
            width: 35
            height: 75
        }

        footer: Rectangle {
            color: "red"
            width: 35
            height: root.height - lv.count * 75
            TapHandler { onTapped: lv.model.append({}) }
        }

        width: 35
        height: 640
    }

    Rectangle {
        width: 35
        height: lv.contentHeight - lv.footerItem.height
        color: "blue"
    }
}