It is a TV limitation. No TV supports playback of TrueHD audio via a TV based app.
If you play a TrueHD audio track using a Plex SmartTV app it will be transcoded by Plex Media Server. Any Atmos information is lost in the process.
The Plex app will passthrough Dolby Digital Plus + Atmos (EAC3 Atmos) to your soundbar. That format is used by the streaming services (Apple, Netflix, etc) and is supported by the TV.
If you want 4K HDR video and TrueHD + Atmos, you should acquire a Nvidia Shield Pro or Xbox One and attach it to an HMDI Input on the Samsung soundbar. The LG TV blocks passthrough of dts audio, so attaching the Shield/Xbox to the TV will result in Plex transcoding dts audio.
Also, do not purchase the Shield “tube” model. It has problems with high bit rate media, such as 4K HDR movie rips. If you buy a Shield, get the Pro model.
See this thread for additional details:
