My configuration is as follow:
Roku Premiere -> HDMI splitter --> Epson Projector
--> Denon AVR 7.1 channels Dolby etc.
Depends on who the Roku device detects first, it downgrades the audio capability to stereo, probably because it identifies the projector as a stereo capability (although it has no speakers). If I connect the AVR first, it might play the first title in 5.1 but then changes down to stereo.
How can I force Roku to continue and stream in 5.1ch format ?
I use it mainly for Netflix and Plex.
You could try switching the audio from Auto to another setting, but it might not stick. My guess would be you either need a different/better HDMI splitter or don't power the projector on until the AVR and Roku have sync'd.
Is there some reason you're using the splitter, rather than connecting the Roku directly to the AVR? That would eliminate the issue.