Thanks @sixones,
This is the strange thing as nothing has changed from a hardware perspective. The catalyst here is likely changing the shield audio to ‘selected’ removing DTS due to the Sonos Arc not being compatible. Sonos recently updated the Arc firmware to accept multi-channel PCM, this allows Plex to play DTS movies and accept the PMC sound.
I think it was mentioned in the thread earlier that people did have issues when changing this option. I can absolutely confirm that the 8.9.0.21577 build overcame this and passed through the True HD/Atmos with the setting enabled.
Tv is LG C9 Oled with eArc connected via HDMI 2.1 to Sonos Arc and Nvidia Shield pro.
I’m on the latest Plex server beta ( Version 1.21.0.3711), I think the Android beta updated without me manually downloading which has broken things.
To summarise:
All hardware and passthrough settings did work for TrueHd when I was on 8.9.0.21577 - this is with the Shield audio output set to ‘selected’. But now when I play a TrueHD Atmos track the audio is transcoded to multichannel PCM and I lose the Atmos 
I assume it’s not easy to just look at the revision differences on the betas as we have had several land since 8.9.0.21577 ?
Thanks again
