this might help @ Plex, 4k, transcoding, and you - aka the rules of 4k - a FAQ
but to answer your question, the xbox has problems with the HD audio.
if you choose a different audio stream (5.1 or stereo) and make sure subtitles are disabled, then (I believe) the xbox should play fine.
as far as another shield client, if you do not have an AV system in the other room, the same applies as above.
to play 4k/hdr without transcoding, you need HD audio support (compatible av receiver), or you need to use non hd audio (5.1 or stereo).