I have a Roku 4 and an Onkyo TX-NR696. I just did a test with Netflix, and it's getting DD+ 5.1 with no problems. The Roku is directly connected to the Onkyo.
Do you have more than 1 output out of the receiver?
I use a 4x4 matrix switcher to connect Roku to 3 rooms, 2 of the devices are Stereo (normal TVs), and 3rd is Living room receiver with 5.1. Roku is connected directly to the switcher. Ive been using this setup for almost 10 years without any issues until this 10.0 update. Ive used it without problems with non roku devices as well as all software releases of Roku since 2015.
There was some change to the logic of DD signal in 10.0 update. Roku is getting confused by device detection. It should not be doing any kind of detection if I am forcing DD/DTS in audio settings, Auto is for that.
Whatever logic DTS 5.1/Stereo use works just fine on 10.0, and logic used by DD 5.1 prior to this update 10.0 worked fine as well.
In 10.0 update DD 5.1 logic was changed, and I think its a bug, because if you plug in an analyzer into the HDMI you can see that its sending garbled signal with an invalid DD signature. This signal is never valid, so Roku should never send it under any circumstances. Because it will just be clicks and pops on any device you play it on.
Roku's dev team needs to either let us roll back (I tried via the aforementioned guide but my only option is current 10.0 version), roll back the logic and release a patch build, or give us an option in audio menu to force DD 5.1 that disables any kind of detection from Roku, just force pipe out 5.1 DD signal regardless of whats on the other end. Call it an advanced option or something.
As I said, DTS 5.1 continues to work flawlessly under the same conditions. The issue is specific to changes in DD 5.1 binary/code in 10.0 release.