I tried the YCbCr420 color format workaround and it works perfectly. So it could really be that our HDMI cables are not up to snuff and lowering the bandwidth required makes them usable. I’m going to try the PaintNothing/frankyfife OGL Buffer-Flipping Mode to OGL FORCE BLIT to ON
"I managed to fix this by discovering a setting using nvidia inspector.
If you go to the retroarch profile. Scroll down to the “Other” section and look for an entry called “Buffer-flipping mode” you will see that it’s set to “OGL FORCE BLIT OFF”
Change this to “FORCE BLIT ON”"
Update: Just for information sake in case the devs are working on an internal permanent fix for this, before I reached to this point I was experimenting with different settings in the nVidia control panel 3D settings and in RetroArch. I noticed that if I set Triple Buffering to On, that settled the black screen flickering somewhat in the RetroArch menu, however it would return when I launched a game. I then started experimenting with different Frame Delay settings in RetroArch and setting this to 15 caused the black screen flickering to stop at the expense of stretched, garbled audio.
The PaintNothing/frankyfife OGL Buffer-Flipping Mode to OGL FORCE BLIT to ON works perfectly though and negates the use of the second best workaround of forcing YCbCr420 and the third best workaround of remaining on 411.70!