Dolby Vision Profile 8.1 with HDR10+ not working on LG webOS client

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A mediainfo output or ffprobe might prove me wrong, but there doesn't appear to be HDR fallback metadata that I can see. First, webOS TVs don't like DV in MKV files, so you would want to manually remux those to MP4 with external subs if you want to keep DV. Otherwise, webOS client will always try to fall back to HDR.

Nothing is being transcoded here, so that's probably the culprit. Do you have hardware acceleration or anything enabled? Can you try one of the non-working media and try to trigger a transcode? I believe you can do this with the settings in the webOS player (Quality: Auto --> something lower than the bitrate of the media).