Are you adding something not on the source? Source would be 4:2:0 unless something new being made I'm not aware of. Are you running games on your 18gbps RS500 ?
Chroma upscaling. Yes.
Uhd Br only has 1920x1080 chroma channel at 4:2:0. Upscaling it to 4:4:4 makes the chroma channel 4k.
Google it, its real. Its also a major reason a lot of people here use MadVR on an HTPC. Which I also run at the same resolution and bit rate.