Noise evident on 4k ripping

[Acehit]

All, I am new to 4k movie ripping, I am currently using FFMPEG to decode my 4k movies, I am selecting x265, and under HDR10 settings to switch to SDR as I want to downscale these movies to 1080p and watch them on my TV. I am selecting aspect ratio to 1920x 1080, Desaturation is 2 and setting is Mobius.

Now this issue is only visible on older movies, but I am noticing a lot of noise on some of my rips. I have attached a screenshot to show you all what I mean. Now I see 4k downscaled movies uploaded on Youtube all the time, even on older movies, and they don't have this issue, so there must be a way around it. Does anyone know what I can do about this please?