Please test following update: http://jernej.libreelec.tv/test/vp9/ If it works, I'll PR fix/workaround.
Yes, I can confirm that this update fixes my problems. All my test movies are working now. Thanks a lot, awesome work. My family will love it.
Side note: only 4K (4000x2250) movie was to slow. Music was correct but video was slow, hanging and jumping. For me that is no issue, as I don't have a 4K TV anyway.