It appears that the problem is only with Dolby D+, either manually set or auto detected. A step down to Dolby D got me going. I mean that you don't have to step al the way down to PCM Stereo. Dolby D,DTS might work just fine.
I tried stepping down to Dolby, DTS and still had the problem. But once I dropped to Dolby, the audio drop out problem went away. Since it is still surround sound, I can live with it. My setup is HDMI ARC to my receiver.
Roku disrespects it customer by making them jump through all kinds hoops to complain about the ARC not working and forcing us to only get Stereo....on our $1000 receiver....stereo was invented in 1960...where are you making us go back to stereo..