I convert the mkv direct to ProRes4444 with ffmpeg. I’m not at home now so I can’t share my command line. I can do that when I get home.

It’s important that force 10bit when you do it. The default with ffmpeg is 8bit. That could be the issue you are having.

I never work direct with an x264 or x265 because it’s always very sluggish and it’ll often show the wrong frame when scrubbing back and forth. In ProRes every frame is a key frame so the frame displayed is always accurate.

Edit: what I use

ffmpeg -hide_banner -i /path/to/file/ESB-HDR.mkv -an -vf “format=yuv444p10le” -c:v prores_ks -pix_fmt yuv444p10le -profile:v 4 -vendor apl0 -r 24000/1001 -force_fps -movflags +write_colr -colorspace bt709 -color_primaries bt709 -color_trc bt709 -report ESB-HDR-ProRes.mov

The 10le in those two places tell ffmpeg to use 10bit instead of the default 8bit