I think it is a hardware-related thing, you can't change to nv12 if the hardware you are connecting from does not support it.
For example, grabbing video from a second board HDMI 1920x1080(out) to HDMI (in) only supports GBR, but grabbing video from my 8K TV set i can have NV12.
For example, grabbing video from a second board HDMI 1920x1080(out) to HDMI (in) only supports GBR, but grabbing video from my 8K TV set i can have NV12.