[benwaggoner]

Are you playing the video back on a 10-bit display through a >8-bit display pipeline? If output is 8-bit, than the dithering of the display device can cause issues.

Is the source 10-bit? In general HEVC has less of a quality gap between 8-bit and 10-bit encodes with 8-bit sources than H.264 had. I recommend using 10-bit encoding only when you have 10-bit sources. 8-bit is quite a bit faster to encode and quite a bit more compatible, particularly on mobile devices.