It used to happen when I upscaled 1080P to 4K as well, but not anymore. It’s placed over most textures, but is most noticeable on human skin.
Anyone who knows how to prevent this from happening? If needed, I can post a screenshot to show an example of it happening, but I don’t know what image hosting sites are allowed and imgur isn’t, apparently.
What model are you using? This will occur when the AI does not have enough detail to work with and latches onto artifacts or areas in the input file that look like “detail.”