The list of screens returned when calling QGUIApplication::screens() changes its order, roughly every 5 reboots in my case.

On my desktop with 2 screens connected, I rebooted several times and start an application containing the following codesnippet once every time:

QString filename = QLatin1String("<some filename>");
QFile file(filename);
if (file.open(QIODevice::ReadWrite | QIODevice::Append)) {
    QTextStream stream(&file);
    for (auto it : QGuiApplication::screens())
        stream << "Data: " << qPrintable(it->name()) << qPrintable(it->model()) << "\n";
}

No other changes to settings or hardware were made between reboots. The produced text file has one entry for each screen for every line for every reboot, and their order switches every couple reboots.

This is also causing bugs downstream, e.g. at KDE, where this bug has been reported 7 years ago already.