I have a Premiere 3920X. It won't show 5.1 available for Netflix and Disney+, but will show it on Amazon Prime.
I have an Onkyo TX-SX393 AVR. It will encode DD+. Hook up is Roku HDMI to AVR and HDMI out to the TV. I have set the audio setting on the Roku to AUTOMATIC on all the surround sound setting. I've even tried setting them at specific DD or DTS and it still won't show 5.1.
The Roku was updated on today with 10.1. It wouldn't work with previous versions either.