QQuickPaintedItem resize is not working as expected

LorenDB

I'm writing an app that uses a QQuickPaintedItem to render to a QML object from C++. However, my code is not working as expected. The C++ code below is a complete, working example of what is going wrong:

#include <QGuiApplication>
#include <QQmlApplicationEngine>

#include <QQuickPaintedItem>
#include <QPainter>

class MyQuickItem : public QQuickPaintedItem
{
    Q_OBJECT

public:
    explicit MyQuickItem(QQuickItem *parent = nullptr)
        : QQuickPaintedItem{parent}
    {}

    virtual void paint(QPainter *painter)
    {
        painter->fillRect(boundingRect(), QGradient::SpaceShift);
    }
};
#include "main.moc"

int main(int argc, char *argv[])
{
#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif

    QGuiApplication app(argc, argv);

    qmlRegisterType<MyQuickItem>("MyApp", 1, 0, "MyQuickItem");

    QQmlApplicationEngine engine;    engine.loadData(R"(
import QtQuick 2.15
import QtQuick.Window 2.15
import MyApp 1.0

Window {
    width: 500
    height: 500
    visible: true
    title: "test"

    MyQuickItem {
        anchors.fill: parent
    }
}
)");

    return app.exec();
}

At first, the application acts as expected: it creates a window filled with a gradient. However, resizing the window resizes the MyQuickItem, which results in some strange behavior. If the MyQuickItem is resized to a size larger than its initial size, it goes blank. Otherwise, it renders more or less accurately to only part of itself (apparently scaling the output to a smaller size, according to my experimentation) while leaving artifacts all over the rest of itself.

Images of what is going on can be found here.

I'm running openSUSE Tumbleweed; I tested on Qt 5.15.2 and 6.1.2.