Win toast / balloon notifications stopped working recently on windows 10 with Qt

Question

To me it seems windows balloon messages stopped working completely some weeks ago on windows 10. In August 2019 the following code based on Qt could be used to post a notification in the windows action center, and letting a toast pop up with that notification:

#include <QApplication>
#include <QSystemTrayIcon>

int main(int argc, char **argv)
{
    QApplication app( argc, argv );

    //create a notification icon, and post a test notification
    QSystemTrayIcon *trayTest = new QSystemTrayIcon();
    trayTest->setIcon( QIcon( "path-to-some-icon-resource" ) );
    trayTest->show();
    trayTest->showMessage( "Hello", "world" );

    return app.exec();
}

But now out of the blue QSystemTrayIcon::showMessage stopped having any effect at all, and nothing is showing up anywhere. A systray application at work is using that method, and windows notifications stopped working on all systems (with windows 10 installed). QSystemTrayIcon::showMessage isn't working with Qt 5.7, Qt 5.12 and Qt 5.13. I had a look on the source of Qt 5.7, and internally Shell_NotifyIcon is called in qsystemtrayicon_win.cpp:

bool QSystemTrayIconSys::showMessage(const QString &title, const QString &message, QSystemTrayIcon::MessageIcon type, uint uSecs)
{
    NOTIFYICONDATA tnd;
    memset(&tnd, 0, notifyIconSize);
    qStringToLimitedWCharArray(message, tnd.szInfo, 256);
    qStringToLimitedWCharArray(title, tnd.szInfoTitle, 64);

    tnd.uID = q_uNOTIFYICONID;
    tnd.dwInfoFlags = iconFlag(type);
    tnd.cbSize = notifyIconSize;
    tnd.hWnd = m_hwnd;
    tnd.uTimeout = uSecs;
    tnd.uFlags = NIF_INFO | NIF_SHOWTIP;

    return Shell_NotifyIcon(NIM_MODIFY, &tnd);
}

Shell_NotifyIcon is used across different versions of Qt for the windows implementation of showMessage. I verified that Shell_NotifyIcon was called when calling QSystemTrayIcon::showMessage under Qt 5.7, it returned 1 like it should if no errors had occurred, and no notification was showing up. Playing around with the "Focus assist" and "Notifications & actions" settings had no effect.

Have there been windows updates recently modifying the behavior of windows notifications? Is it only possible to post notifications with some new native windows 10 apis? I couldn't find anything concerning windows updates and notifications not showing anymore.