I have an MSI Optix G27C2

In Driver version 456.71 this monitor is no longer able to display color format RGB at 120Hz the option is no longer available. In 144Hz this monitor does display RGB. The only output option in 120Hz is YCbCr422

In driver version 446.14 the same monitor at 120Hz has the display color format RGB available.

YCbCr422 has a lot of color artifacting so it's not really usable.
 
And to answer the question of "why are you using 120Hz?" videos look smoother on the desktop it reduces judder.

Best to report that to Nvidia as a bug. There have been quite a few modifications to the drivers for the 30 series and certain Gsync compatible monitors lately.

Good point submitted a bug report to the nvidia driver feedback form

Locking this thread as you already have one open for this same issue.