So, I was referring to a Atmos manual. Just verifying there is no reason from Dolby that the speaker would be omitted from the processing.
But yes, you have the problem down, however the processor isn't behaving how you describe. In fact, if it did behave that way, I probably would have looked past it even though the processor should be using all the speakers it supports with Object-based metadata. (not Dolby Surround Upmixer). The problem is made much worse in that it ignores the rear speaker, and treats the side surrounds as rears, then pushes most of the side surround audio to the front channels. So 5.1 content is mostly all on the front soundstage. The height channels seem un-affected.
You'd think as you said, that 5.1.4 is how it would behave, and the back surround data is being sent to either heights or the side surrounds to get it close to the back as possible since it doesn't think I have any back surrounds. Instead it treats my system like I have 2 back surrounds, and the LCR, but no side surrounds, and the side surrounds are playing the back surround signals. I know the Denon is supposed to support the speaker placement, but my guess is they have a serious (to me) bug in their Atmos implementation that is throwing ALL Atmos content way way off with that setting enabled. So it's worse than just not supporting the speaker with Atmos content, it breaks Atmos soundstage completely. To get it better, I either have to delete the speaker from the settings and set a 5.2.4 manually, or use it as a Left surround back and have a dead spot in the soundstage since I don't have a right. Then I have to re-calibrate if I want to re-add that speaker for DTS:X or DTS:Nerual content.
The point with the Atmos manual, is it is a supported Atmos speaker position, as well as a supported Denon speaker position. I don't need it to be a "back Dolby", just need it to be a 1 speaker back surround. If I'm not mistaken, "back Dolby" would be a pair of back up-firing "Dolby enabled" speakers, correct?
What receiver page is that by the way? Didn't look like my manual, but the options are just like that. So I have Surround Back set to 1 spkr, and it is plugged into the left side correctly.
@Gmoney
I didn't think Yamaha receivers supported a center back channel, at least not at the 11.1 level.