Follow the directions below. Under the Audio settings on the Roku system settings, set Audio Mode to "Auto", and set HDMI to "Dolby D+".
What is the make and model of your AV receiver? We can double check to make sure it supports Dolby D+, which is different than Dolby D.
I tried all your suggestions but none of them worked. A coworker lend me a Roku Ultra 4640X and it works very good with 5.1 DD+.
Software version is still on 8.1.0 build 4137-29. I just hope that if it takes the upgrade it won't continue to play DD+ audio.