I have had a few (very few) videos that very rarely stutter and so far the fix I have found appears to be working.
The fix relates to a value in the amlogic kernel (“dynamic_buf_num_margin”). There was a fix introduced in CoreELEC 9.0 that would set this value to 16 if you had enabled it in Settings-System-CoreELEC and the video was 1080p resolution or lower. It was called “Enable HEVC seek workaround” or similar. The default value was 8. In Matrix 19.0 (and I think in v9.2.6), the default value is now 7.
With my few problem videos (and now your sample), they all stutter in the same place with that value left alone. If I set it to 16, the stutter disappears. So far I have experienced no side effects changing the value to 16. Since v9.2.6 that option has been removed from the gui, so I make use of the “autostart.sh” script file to set this value at startup of CE. This file should be placed in “/storage/.config” directory within CoreELEC. I also ensure that the files properties are RWX for all groups (I do this via WinSCP program).
The command to insert into this text file is:-
echo 16 > /sys/module/amvdec_h265/parameters/dynamic_buf_num_margin
One thread that mentions this is here:-Stuttering with certain encodes (CVideoPlayerAudio::Process - stream stalled)
I don’t have any 4k videos, all mine are 1080p or 720p. Still, I think this is worth trying. Your sample plays perfectly with this value set to 16, but with the default value of 7, I see exactly the same results as you quoted.
I would have a go with changing the “dynamic_buf_num_margin” value and see if that fixes it for you.
PS. I actually did manage to find a couple of 4k HDR videos and with the very quick testing I have done, these all seem to playback fine and skip forwards and backwards without issue (so far…). This is with the “dynamic_buf_num” still set at 16 - ymmv.