OK - Bug reports.
The previous app didn’t worry about figuring out sound until a video was casted. This meant that if my amplifier was not on at the point that the Plex web app was cast - that was fine, as long as I turned it on before trying to cast something with 5ch sound. The new app appears to negotiate what sound profiles are supported by the equipment as soon as it loads, and even if I turn the amplifier on before casting something with 5ch sound - it is transcoded to 2ch. Disconnecting from the Chromecast and recasting once the amplifier is turned on - brings back 5ch sound.
I casted Harry Potter and it was all fine - according to Tautilus it was Direct Play. I then selected an SRT subtitle stream and it became Direct Stream because it was transcoding the container from MP4 to MKV and the subtitles from SRT to ASS. That was fine, that’s not hurting the CPU. I then skipped to about 45 minutes in to the video and suddenly Video, Audio and Subtitles were all Transcoding - (Subtitles changed to BURN) so clearly, it doesn’t like seeking through the video while subtitles are on.
I quit the app and recasted - and haven’t been able to replicate number 2 yet, but if I do - I’ll get some logs for you.