LG TVs don’t support any Dolby Vision in MKV files and only support some Dolby Vision profiles in MP4 files.
If you play a not supported file that has DV and HDR10 it will show the HDR10 version, but if your file only contains DV it will show a wrong picture.
See here on how to convert MKV to MP4 to be able to watch Dolby Vision: What's the status of Dolby Vision + Atmos on Plex? (re: LG WebOS, Nvidia Shield, Apple TV) - #290 by mpiva
