QtSerialPort::write does not send any data

We have a Qt 5.15.2 application and send data via QtSerialPort::write

d->SerialPort->write("T\r\n");

This works. Now we switched to Qt 6.5.2 and noticed, that this code does not send any data. Also calling the function multiple times or with a longer data string, does not trigger sending of data. The only solution was to modify the code in the following way:

d->SerialPort->write("T\r\n");
d->SerialPort->waitForBytesWritten(1);

This triggers sending of data and then the driver works like in Qt 5.15.2. The SerialPort object runs in its own worker thread.

The question is: Is this a bug or a feature?