Hello LibreElec Users,
i have a LG 55EF9509 and my best friend a LG 55B6 OLED TV and we both use LibreElec (Krypton) with a S905x chip. When we start HDR content, we have to push the "back" Button on the remote and restart the video to see correct HDR with the real colors. Otherwise the colors are not correct in HDR mode and look very washed out. This fix with the "back" button was very disappointing and go on my nervs but now i find a nice workaround 
=> Switch (under Settings->System) the Refresh rate to 24hz and the HDR bug is gone. The menus will not look as smooth like on 50hz or 60hz, but the HDR bug is fixed. I think the refreshrate switch from 60hz/50hz to 24hz causes problems together with the activation of HDR mode.
If the "microstuttering" menu animations are a big pain in your eyes, then you can disable the animations....
In Terminal you have to edit the file:
~/.kodi/addons/skin.********/addon.xml (***** you used skin)
change effectslowdown="1" to "0"
After a restart, you can enjoy Kodi without HDR bug and with instant click menus without animations