A friend of mine passed onto me that he has problems with LibreELEC playing on a RPi2, with the HW codec keys installed (if that's relevant), a mkv container file with VP9 and OPUS inside.
The same issue occurs on my install (and has done for a number of releases), I'm running 8.2.5.
The symptoms are the video plays very slowly (practically frame by frame) and the audio initially sounds normal, but seems to get upset eventually.
There is nothing in the log file that is complains during this playback. The file plays fine on VLC.
Any ideas? I'll try and attach an example file or link to it.