Looks like my Panasonic EX730 may have some of the same issues as other platforms, enable Dolby/DTS and I get no audio over ARC, disable them and I get stereo sound only. Netflix happily outputs 5.1 just fine.
When the options are disabled the Plex server is transcoding the audio to AAC 5.1 from DTS or AC3, but the TV only outputs 2 channels to the receiver. When enabled it says it's using Direct Stream.
FWIW the Xbox One and Shield TV both output everything perfectly but as the Shield TV is purely a Plex device for me, it'd be good to be able to retire it.
Any ideas? Is this a bug/feature?
Edit: Tried the optical output instead, exactly the same result.