Or the issue is such a low priority that they never got around to it. I’ve had the same thing happen with some of my support requests.
You mentioned Quad 4.0 to 7.1 isn’t working either. Unfortunately, I only have a 5.1 system (upmixing 4.0 to 5.1 works). It would be interesting for someone else with a 7.1 system to do the test I did to see if 4.0 to 7.1 upmixing is broken for them as well.
My meaning:
For all my multichannel sources, I’ve also the correspondenting stereo source and so for me there isn’t a need for a downmix.
But on the other side, you’ve set in Roon the output (for multichannel) to 5.1 or 7.1 and then for example a 5.0 source will remixed to 5.1 or 7.1, what’s not fine for me.
For wishes like 4.0 → 5.1, I can only say: There are no general rules for this and what I want to have as output should be bitperfect and not remixed.
I have a 6.0 in the living room. Before the problem arised the surround channels form Quad files were routed to the Surround back channels according to their Knoledge base. When the problem with downmixing started the same Surround channels were routed to the Surround side channels.
And the downmix to stereo did not work any longer.
So , in your case when playing Quad to 5.1 output the centre speaker and the Subwoofer are involved too? What is your source. Are these flac files?
Yes, it probably would be the best to have Surround Albums in Stereo too.
But sometimes even that is not possible and would actually only be a Workaround of a foult in the Software which is still not fixed.
As for as I know, a 5.0 will not be upmixed to 7.1, even if you have set it up in Roon as 7.1. The Surround back files will not be audible. At least that is the case in my set-up.
From what I understand when playing Quad 4.0 on a 5.1 system like mine, the sub and center channels are not active since the 4.0 signal doesn’t contain these channels. Roon sends the two front channels in the Quad signal to the front speakers, and the two rear channels in the Quad signal to the surround speakers. In my testing, this is how it worked.
Now that I think about it, I shouldn’t be using the term “upmixing” since I don’t believe this is what Roon does. Roon is simply remapping the existing channels in the original signal to the most appropriate channels in one’s system. It’s not doing any “upmixing” to create additional channels not in the original signal (such as sub, center, etc).
Unfortunately, I don’t have any Quad 4.0 content other than the channel identification test album I referenced earlier in the thread (they are flac files). Nor do I have the ability to test if the rear channels in a Quad 4.0 signal are being remapped as described in the Roon KB to a 7.1 setup.
We’re confusing a few things 
Channel mapping doesn’t require DSP and I use DSP in this context as signal processing. Channel mapping just moves the PCM that was in channel 3 to channel 5 or whatever. It’s literally just finding the start of that channels bitstream and moving its location.
Mapping quad or 4 channel to 5.1 is simply putting the right bit stream into the right channel location.
Now, if you want to “mix” you need DSP. That is, I want to put the bass at some xover frequency into the .1 channel and remove it from the others. That requires DSP and configuration of that DSP. Or I want to take rear channels and “mix” them into the L / R channels for 2 channel stereo. This mixing of other channels to L / R is more straightforward so the configuration can be “baked” into the operation. However, it still requires a baked configuration (volume matching, etc.). The “downmix” option in Roon is an example of this baked DSP configuration.
A lot of this “mixing” requires configuration or at least common multichannel mapping so you don’t end-up mixing the wrong channel. I speculated somewhere else that quad → 2.0 might be broken because they are mixing the wrong surround channels which means they are mixing silence into L/R. Quad and 4.0 don’t have a clear standard. There are tools to do this mixing yourself, letting you define which channels go where, and then you can just load the 2.0 output as files in Roon.
Yes, you are right. That was exactly what I have done in the past as a Workaround: using the Procedural Equalizer in the DSP to manually route the Surrounds Sides and Surround Backs to the Fronts, and setting the Headroom Managment to -3 to prevent clipping. That worked mostly well, but some files just did not sound right. And I never knew whether the routing itself should always occur at 0 DB.
The other problem arrised, when switching to a track with MQA, the playback stopped with an error. The DSP first had to be switched off again and that was not something I wanted to do.
Actually I played with this setting in my Surround system as well. Sending Surround Sides to the Backs with -6 db for “fake upmixing”. But that was not ment to be a serious setting. I was just curious about it.
…and I am quite sure that you are right here again, as it was exactly what happened when the problem arised. On the Quad files the Surround Backs suddenly went to the Surround Sides, hence the silence in the Stereo downmix. The question is: what have they done during the update to 17.610