It doesn’t occur on Windows 8.1 for me, and I think it is a problem or limitation with your audio device rather than a Windows version issue.
What exactly happens when you ask for a mono recording of a stereo input depends on the sound device in any case. Built-in sound devices will tend to give you a mono mix. External or PCI devices will tend to only give you the left channel.
Tracks > Stereo Track to Mono will give you a mono mix.