Interesting, fair enough, that makes sense. So your receiver was getting a 5.1 signal, but it really did just ignore half the channels when you set it to output 2.1.
That’s not the problem I think most people here are complaining about though, which is sound mixing / dynamic range / editing making speech too quiet, rather than having the wrong settings.
You can opt out of audiobooks and save the extra cost they added the other month. https://www.moneysavingexpert.com/news/2024/05/spotify-saving-basic-plan-audiobooks/?source=CRM-MSETIP-84b484ca1c047788624