I see this on the original output from UHD disks, as such it seems to be an artifact in the original.
Some producers of consumer content filter and smooth the video to hide film grain.
This works pretty well for 1080p where the pixels tend to be bigger than the film grain. It works less well for 4K where the grain is on the same order as the pixel size.
Many 4K producers use the convenient rationalization that the limitations of the original media represent ‘artistic intent’. I am sure somebody who could afford better quality film made it grainy on purpose, but that is only one. Low light is another complication that contributes to graininess since faster film always produces more grain, all other things being equal.
Though it is technically difficult to do, I would much prefer digital filtering to remove as much grain as possible to the ‘artistic intent’ excuse, but tastes vary.
Your other choice is to buy 4K UHD only for recent titles where digital cameras were used and once again the source material is much better than what you have at home.