Using QSerialPort with standard/enhanced silicon labs dual CP210X gives 'device attached to the system is not functioning' error

Silicon Labs Dual CP210x USB to UART bridge is a device with 2 UART interfaces, one is shown as 'Standard COM port' and the other as 'Enhanced COM port' in device manager on Windows 7.

When trying to open the Enhanced COM port using QSerialPort, it opens fine, however when trying to open the Standard COM port, Qt reports an error 'Error: A device attached to the system is not functioning'.
This however is not true, if I use an alternative serial program (the one I am using is written in borland C) then the port opens fine without issue. After opening the port with the other program and closing it, QSerialPort is then able to open the serial port fine and communicate, until the device is unplugged and plugged back in whereby the same process needs to be repeated.

I understand that replicating this scenario might be difficult so I can spare one prototype development to a developer that needs to test against this if required.