Exactly that. 120hz at 4k hdr can only be done by using 422 on ps5 due to i's hdmi chip limitation at 32gbps.
It's a ps5 limitation but don't let it bother you. There shouldn't be a visible image quality difference outside of test patter from rtings.
Also. What everyone is saying about switching c1/c2 to PC input label is true. I tested it.
If you are using default c2 console input label, even at 60hz, despite the green button(you click 7 times on remote to view details) menu reporting RGB HDR, it is still 422 underneath.
In order to be able to display full real 60hz hdr RGB, a pc label must be used on lg c1/c2.
But at 120hz, it makes zero difference. Only at 60hz but you would still not be able to find any image quality difference.
You can get test pattern here. You do need to put that image inside 4k picture though, so that ps5 media gallery is not scaling it.
What is Chroma Subsampling and where is this visible? Chroma subsampling is a type of compression that reduces the color information in a signal in favor of luminance data.
www.rtings.com