Assigning QVector<QString> to Repeater's model property results in count being 0

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QAbstractListModel>
#include <QQmlContext>
#include <QKeyEvent>
#include <QDebug>
#include <QAbstractListModel>

class ItemData : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QVector<QString> roundNames READ roundNamesVector CONSTANT FINAL)

public:
    ItemData(QObject *parent = nullptr) : QObject(parent) {}

    QVector<QString> roundNamesVector() const {
        return QVector<QString>(mRoundNames.constBegin(), mRoundNames.constEnd());
    }

    QSet<QString> mRoundNames;
};

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;

    qmlRegisterAnonymousType<ItemData>("App", 1);

    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);
    engine.load(url);

    ItemData itemData;
    itemData.mRoundNames.insert(QLatin1String("Test"));
    engine.rootObjects().first()->setProperty("itemData", QVariant::fromValue(&itemData));

    return app.exec();
}

#include "main.moc"

import QtQuick 2.14
import QtQuick.Window 2.14

Window {
    id: root
    width: 300
    height: 480
    title: "count: " + repeater.count + " roundNames: " + (itemData ? itemData.roundNames : [])
    visible: true

    property var itemData

    Flow {
        anchors.fill: parent

        Repeater {
            id: repeater
            model: root.itemData ? root.itemData.roundNames : null
            delegate: Text {
                text: modelData
            }
        }
    }
}