Missing touchUngrabEvent callback when Flickable steals press

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQuickPaintedItem>
#include <QPainter>

class TouchButton : public QQuickPaintedItem
{
    Q_OBJECT
public:
    TouchButton(QQuickItem *parent = nullptr)
        : QQuickPaintedItem(parent) , m_pressed(false), m_touchId(-1)
    {
    }

    void paint(QPainter *painter) override
    {
        painter->setOpacity(0.75);
        painter->setRenderHint(QPainter::Antialiasing);

        painter->setBrush(m_pressed ? Qt::darkBlue : Qt::blue);
        painter->drawRoundedRect(boundingRect(), width() / 10, width() / 10);

        painter->setPen(Qt::white);
        painter->drawText(boundingRect(), Qt::AlignCenter, "Press\nand\ndrag");
    }

protected:
    bool event(QEvent *event) override
    {
        qDebug() << event;
        return QQuickPaintedItem::event(event);
    }

    void touchEvent(QTouchEvent *event) override
    {
        switch (event->type()) {
        case QEvent::TouchBegin:
            if (m_touchId == -1) {
                m_pressed = true;
                m_touchId = event->touchPoints().first().id();
                update();
            }
            break;

        case QEvent::TouchUpdate:
            for (const QTouchEvent::TouchPoint &point : event->touchPoints()) {
                if (point.id() != m_touchId)
                    continue;

                switch (point.state()) {
                case Qt::TouchPointPressed:
                    m_pressed = true;
                    update();
                    break;
                case Qt::TouchPointReleased:
                    m_pressed = false;
                    update();
                    break;
                default:
                    break;
                }
            }
            break;

        case QEvent::TouchEnd:
            for (const QTouchEvent::TouchPoint &point : event->touchPoints()) {
                if (point.id() == m_touchId) {
                    m_pressed = false;
                    m_touchId = -1;
                    update();
                }
            }
            break;

        case QEvent::TouchCancel:
            m_pressed = false;
            m_touchId = -1;
            update();
            break;

        default:
            QQuickPaintedItem::touchEvent(event);
            break;
        }
    }

    void touchUngrabEvent() override
    {
        m_pressed = false;
        m_touchId = -1;
        update();
    }

private:
    bool m_pressed;
    int m_touchId;
};

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    qmlRegisterType<TouchButton>("TouchButton", 1, 0, "TouchButton");

    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    return app.exec();
}

#include "main.moc"

import QtQuick 2.5
import QtQuick.Window 2.2
import TouchButton 1.0

Window {
    id: window
    width: 640
    height: 480
    visible: true

    Flickable {
        anchors.fill: parent
        contentWidth: parent.width * 1.5
        contentHeight: parent.height * 1.5

        TouchButton {
            width: parent.width / 4
            height: parent.height / 4
            anchors.centerIn: parent
        }
    }
}