HDR content can not properly be transcoded. This requires 2 copies of each file SDR & HDR. Client that require transcoding must manually select the SDR version.
Emby supports multi-versions, add an option that forces the SDR version anytime a transcode request is made.
This feature stays relevant even after HDR tone mapping for transcodes is released. The superior image quality option is to never transcode HDR content. I still suggest this is a admin library option, there will be admins who would not want this behavior.