I have issues with some 1080p or 4k movies on my ATV4K and the last version of Infuse, the movies are on a Synology NAS.
There’s a lot of grain, specifically on black color, (see on the linked picture, it’s supposed to be a black transition screen).
I’ve tried different settings, still the same problem. I’ve watched those movies with Infuse on my iPhone and iPad, no problem at all. The problem doesn’t occur with other apps on my ATV.
The HDMI cable might be faulty, the image is bad only when HDR is enabled on the ATV.
I’ve tried with another HDMI cable, it works fine, even with HDR enabled.