QSerialPort not working

Our application uses QSerialPort to communication with our READ (hardware our company produces).
Since the upgrade to Qt 5.13.1 the communication no longer works. readyRead() is not emitted until a lot of data has been written, but our commands are usually just a few bytes long.
We used Qt 5.9.0 before and all worked fine.
Tested it with Qt 5.12.4 and Qt 5.13.0 - with both at least the Qt Terminal example works fine.

The behavior can easily be reproduced with the Qt Terminal example - it is unusable with Qt 5.13.1.
Our serial port settings are:

	m_serial->setBaudRate(QSerialPort::Baud115200);
	m_serial->setDataBits(QSerialPort::Data8);
	m_serial->setParity(QSerialPort::NoParity);
	m_serial->setStopBits(QSerialPort::OneStop);
	m_serial->setFlowControl(QSerialPort::NoFlowControl);

It appears
Commit: Emit _q_notify only if there's no notification pending
Change-Id: I0ee4522db63844bdd37169d3a31bde2d6f92f383
is a regression.