I am not sure DTS HD Master Audio is supported via HDMI ARC currently. So it explains maybe your issue.
We are talking in this topic about DTS High Résolution Audio witch is not the same. It is a lossy codec whereas DTS Master Audio is a lossless codec. In my case, DTS MA is passing through ok whereas DTS HRA is not (but with Kodi Plex plug-in, it is working). It appears it is a bug in the way the HDMI passthrough is initialized by Plex Android TV player :
