sample rate limit for coax / optical ( S/PDIF ).
Bitstream is shown as the default setting.
The Shield Pro settings: (Advanced Sound Settings) One) Audio Output, Auto or USB (your case).
Two) USB Audio Mode, High Quality Stereo.
Having a 16GB Android TV Shield (updated / no HDR) the online user manual covers up to the latest Shield TV Pro (2019 at least) settings.
My use has been streaming to HD TV no external components (HDMI) and not having tried using Shield as a UHD Audio Streamer.
Something else for me to do ...
One thing you could try is to set up the Shield without the Oppo in the flow ...