Maybe devs know something regarding this issue. Kwiboo ?
This is the main "blocking" issue from my point of view, it seems to happen when mode is changed from one 4:2:0 12-bit mode to another 4:2:0 12-bit mode (and yuv plane is involved). I think LibreELEC-Generic.x86_64-9.80-devel-20200201013402-e776789.img.gz contain a limit to use 8-bit output and if I remember correctly using 4:2:0 50/60hz 8-bit works, it will activate HDR mode on my LG OLED but I am sure colors will not be optimal.
I have not been able to produce a simple reproducible test case (not using kodi) for this 4:2:0 50/60hz distorted image issue.