QSerialPort::bytesWritten() never called on Windows 10

In a class, called FrameWriter, I have a slot that is called by the QSerialPort::bytesWritten() signal. This slot then sends more data to the serial port.

The "FrameWriter" is tested once with a QBuffer, and works fine (Windows, Linux).

This class is used by a other class, called SerialPort.

To unit test my "SerialPort" class, I have a plug that bridges Rx/Tx (pin 2 and 3 on DB9 connector), then simply send data and check them back.

On Linux, with Qt 5.13.0 all works fine. On Windows, QSerialPort::bytesWritten() is never called (Qt 5.13.0 as well as 5.13.1).

To reproduce: bridge Rx/Tx on the serial port. Run the Terminal Example. Select the port, disable the local echo option. In the terminal, type some chars and see them displayed back. For me, on Windows 10, works fine with Qt 5.12.4. With 5.13.x no char is displayed back.