Code
Adapted from https://doc.qt.io/qt-6/qsplashscreen.html#details

static void doLongAndExpensiveInitialization()
{
    int i = 0;
    while (i++ < 5)
        QThread::sleep(1);
}

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QPixmap pixmap(":/splash.png");
    QSplashScreen splash(pixmap);
    splash.show();
    app.processEvents();
    // app.processEvents(); // WORKAROUND: Call processEvents() twice

    doLongAndExpensiveInitialization();

    QMainWindow window;
    window.show();
    splash.finish(&window);
    return app.exec();
}

Expected outcomes
When the app is started, the splash screen shows on screen for 5 seconds, and then disappears as the main window appears.

Actual outcomes
When the app is started, nothing shows for 5 seconds, and then the main window appears.

Workaround
Call processEvents() twice instead of once

Notes

• Tested with both -platform xcb and -platform wayland
• This doesn't seem to affect Windows. In fact, Windows shows the splash screen even without any calls to processEvents()
• Qt 6.2.8 did not have this issue