terrible compression artifacts in shadow regions streaming Netflix apple tv4

I recently rewatched the entire DS9 series on Netflix. I have both an Apple 4K tv 4 and a Roku UItra. I prefer the Apple TV for closed captions becauseI can set the subtitles to be both small and green. With the roku set the same way the closed captions are small but white and more irritating (Netflix app on Roku overrides settings). However on shadow area compression artifacts there is a huge difference between the two streaming devices. The Apple TV with a strong connection to a 5Ghz band on my AC router shows a lot of blobby shadow artifacts whereas the Roku device shows none at all. So, overall the picture quality is considerably superior on the Roku, and only the closed captions superior on the Apple TV. Does anyone have an idea why the compression artifacts are so horrible on the Apple TV? That is my one and only disappointment with the device. '