Sorry to bump a really old thread but this seems to be the only one about stereo upmixing on AMD.
The problem currently seems to be (at least on my RX 6700XT) that it is always upmixing stereo content to fill my 5.1 speakers, when the audio output speaker settings are set to 5.1. When I set the audio output device to stereo, the upmixing stops.
Strangely, when I connect my AVR (Denon X1200W) to the integrated Radeon GPU of my 7600X CPU, there is no upmixing.
There was no way to get the Radeon RX 6700XT to stop upmixing stereo content to fill the amount of speakers. Not even disabling all audio enhancements in advanced audio settings in Windows helped.
It really should be off by default and then the user can decide whether they want audio upmixing / speaker fill.