Try adding the `-pix_fmt yuv420p10le` option (e.g. before `-vf)`. Or only `-pix_fmt +` for using the same pixel format as the input video has..

If it doesn't help, *you obviously have an inappropriate, albeit more common FFmpeg version — only for 8-bit colors.* (Your original video uses 10-bit colors, and FFmpeg will try use the best 8-bit pixel format in this case - with the colors' degradation, of course.)

FFmpeg may be built for 8-bit colors or for 10-bit colors, or — in recent time —  for both of them (for your HEVC encoded video). 

So you need to build (or obtain by other way) version for 8-/10- or 10-bit color depth.