I have the same Yamaha Soundbar. It simply does not like 4K with a 4:4:4 Chroma setting.
Use the 4:2:2 image.
If specific HDR Rips are too dark, then you will have to wait till I get an Ubuntu 16.x Virtual compiler up and running again.
I've upgraded to Ubuntu 18.x and it killed a LOT of previous trouble free LibreELEC 8.2.x compiling.
or disable 444,10bit in Kodi AMLogic settings and Force 4:2:2 by putting this in autostart.sh
echo '422,10bit' > /sys/class/amhdmitx/amhdmitx0/attr
1000nits will be the default in both cases for previously too dark looking 4K HDR Rips
Good to know thanks!
I'm already back on the 8.2.5 422 10-bit image and everything is working as expected again. 4k HDR does look darker than 1080p, I'm not sure I have any non-HDR rips to test the difference. I'll just live with it for now