I ran across a feature request in Plex yesterday quoting Jellyfin and made me think if this has even been implemented in Emby. Evidently Jellyfin has this posted:
https://github.com/jellyfin/jellyfin/pull/7828
https://www.reddit.com/r/jellyfin/comments/v2nudu/dolby_vision_to_sdr_hwa_tonemapping_is_coming/
Plex has told me that they "don't have the license to tone-map Dolby Vision". No idea if that statement makes sense or is fudged to give an excuse.
I'm guessing Emby DOE