I don't know why this happens. However, I have some suggestions of course.
First a little background on Mono serial port and drivers. On Windows we do not use any third-party driver - not even Cypress' driver. What we do is install a INF file that specify our USB CDC Serial port with the systems default USB CDC driver - which is provided by Microsoft.
In the Device Manager you should be able to confirm that by clicking the tab Driver, like this one:
The driver provider should be Microsoft - not cypress or anything else.
If not, you can try our dedicated driver installer package (that installs the INF file) here.
I hope inspecting the driver configuration in device manager sheds some light on the issue.