The optical connection is unidirectional. Unless the AVR is connected to the TV using HDMI, there is no mechanism to determine the audio capabilities. That is why we provide the ARC/SPDIF manual settings so that you can tell us what the AVR can do. Leave the Audio Mode set to Auto though and then the Roku will automatically select the type of audio to send out optical based on what you have set in the ARC/SPDIF setting.
Is this still possible with TCL 6 Series?
I'm having similar issue where it is always stereo. My setup is 65R615 to Onkyo TXSR605 via optical. Not utilizing ARC as receiver doesn't support ARC; however my receiver is connected to HDMI 1 port.
The problem I'm having is that the TCL is always sending stereo output even though I have it set to Auto and should be sending 5.1 for specific media am testing.