I did a new experiment to find out what's going on in FFmpeg with RGB capture
The idea was to inject a pure 4:2:2 video signal into the Intensity Pro 4K and try to capture it with the -decklink tag of the same version of FFmpeg than before, just to show if it is something related to FFmpeg in general or to the RGB feature only.
To do that I use a Blackmagic HDMI to SDI converter that only outputs 4:2:2 video, and then, I re-inject this video in my Intensity Pro 4K capture card (I need to use an intermediate video capture card to convert SDI to HDMI for the Intensity Pro 4K as I don't have SDI to HDMI converter).
You can have a look at the full setup in the picture attached.
Then I tried to capture this signal both ways. First, using -decklink tag and a FFmpeg compiled with the --enable-decklink and then using -dshow on a standard FFmpeg build. Here are the command lines :
ffmpeg.exe -f decklink -bm_v210 1 -format_code "hp60" -i "Intensity Pro 4K" -r 60 -vcodec copy -acodec copy output.avi
ffmpeg.exe -rtbufsize 1500M -f dshow -s 1280x720 -r 60 -i video="Decklink Video Capture (2)":audio="Decklink Audio Capture (2)" -vcodec copy -acodec copy output.avi
It is working in both cases (with 4:2:2 input):).
Of course, if I connect the source directly to the Intensity Pro 4K (RGB) it is not working (I got only black video captured).