unless you have both a TV and RECEIVER with E-ARC, HDMI will not pass HD audio.
HD audio such as atmos/truehd/dts-hd will not work and will be transcoded unless you choose a dd/dts audio track as you have discovered.
this may or may not help, see also @ Plex, 4k, transcoding, and you - aka the rules of 4k - a FAQ
