Well, that took a turn, didn't it?
If your channel level settings were not being kept after powering off the unit, then it had an internal issue and probably needed replacing anyway. It is a "budget" unit and features are missing for cost AND higher end model considerations. The issue with audio of surround channels being output to the surround back channels instead when playing Dolby 5.1 material is strange, but it is what it is. The new Dolby and DTS up mixers cater to object placement with height channels and the surround back channels are now the most expendable when considering configuration of speakers with height channels. It isn't so willy nilly. The manual informed you that info would come from the surround back channels and not the surround channels with Dolby 5.1 signals with a 7.1 configuration and that is only the case if not using the Dolby Surround up mixer and using a "straight" Dolby decode.
Don't confuse the HDMI 2.1spec as a whole with supported features and bandwidth. HDR10+ is not an HDMI 2.1 feature and worked with HDMI 2.0. Onkyo simply left support for it off three ports because it is a "budget" model. There are trade offs when you try to save a buck. HDR10+ is scarcely available on disc and streaming services anyway no matter if streaming devices, receivers and TVs support it. Do not worry about 8K in your lifetime. Services are barely able to get quality, highly compressed 4K HDR to customers now. The bandwidth is not there and will not be there for a long time. There are very few devices supporting 48Gbps at the moment and many may not support it for years and will have to make do with using DSC to squeeze bandwidth hogs into the pipe. Streaming content, and even watching UHD discs, does not require more than the 18Gbps and 4K Enhanced features of HDMI 2.0. Do not be consumed with getting every feature under the sun because there is NO single device on the planet that offers it ALL. And, just as soon as they add new features, they take some old ones away.
Save some of your energy for the complaints you will have about the Denon. There have been many changes in their displays and features over the years. The great front panel display of Denons from years ago is gone and you will see incoming channel info but not output channels unless you bring up the on screen display. I hate the abbreviated display of the input source and sound mode on new models. Center Spread is not adjustable anymore and it is an ON or OFF affair. While supporting legacy analog devices, there are limitations so read that manual from front to back. The S models give up features from the X models. HEOS sucks, period. It was one of a few reasons I returned the S760H for the TX-NR6050. Getting exactly what you need and want in a receiver requires research and often times more money. It became ridiculously expensive to continue to get the kind of build quality and feature set of the 3803 with later model Denons and I just went another way after using a couple of other Denon models after the 3803 and gave Onkyo a shot. It worked out for me, but perhaps not so much for others.