To reproduce:
Set the attribute, Qt::AA_SynthesizeTouchForUnhandledMouseEvents on the QApplication in main.cpp.
Create an instance of a QWidget derived class with e.g. the following definition of event:

bool Widget::event(QEvent *e)
{
  switch (e->type())
  {
    case QEvent::MouseButtonPress:
    case QEvent::MouseButtonRelease:
    case QEvent::MouseButtonDblClick:
    case QEvent::MouseMove:
      qDebug() << "Squeak squeak...";
      e->ignore();
      return false;
    case QEvent::TouchBegin:
    case QEvent::TouchUpdate:
    case QEvent::TouchEnd:
    case QEvent::TouchCancel:
      qDebug() << "Stop touching me!";
      e->accept();
      return true;
    default:
      break;
  }
  return QWidget::event(e);
}

Run the application.
Click on the widget with a mouse.

Expected results:
"Squeak squeak...", followed by, "Stop touching me", as the unused mouse event should be synthesized into a touch event.

Actual results:
Only the "Squeak squeak..." output appears.

I have verified that touch events are accepted by the widget (by using the touch screen).

I have tested with both QWidgets and QQuickItems within QQuickWidgets. Neither worked.

Debugging a minimal example, I noticed a possible problem in QWidgetWindow::handleMouseEvent(QEvent *event).

At line 1771 of qguiapplication.cpp, QGuiApplicationPrivate::processMouseEvent(QWindowSystemInterfacePrivate::MouseEvent *e) calls QGuiApplication::sendSpontaneousEvent(window, &ev). This QMouseEvent pointer, ev eventually get passed down to QWidgetWindow::handleMouseEvent(QEvent *event).

At qwidgetwindow.cpp line 538, it creates a QMouseEvent, translated, which copies most of the original event's data, but takes the windowPos as its localPos. translated then gets sent down the widget hierarchy by a call to QApplication::sendMouseEvent(..). In the situation we're interested in (Nothing accepts the event), translated eventually gets its member, m_accept set to 0.

However, the accepted value of translated is never taken into account once control returns to QWidgetWindow::handleMouseEvent(QEvent *event), thus the original event that was passed into the function still has m_accept set to 1. This means that when control gets back to QGuiApplicationPrivate::processMouseEvent(..), the expression !ev.isAccepted on line 1772 evaluates to false, meaning the code to synthesise touch events is not executed.