When you downmix, you are at the whim of your hardware. Ideally, the sound coming from the left and right speakers would be the correct mix of left, right, and center channels and it would just dump the surround (I don’t know if any of them rematrix it).
I believe that the rear channels are included when downmixing to stereo. If they weren’t, then you would be missing audio that is completely/mostly absent from the front channels in a discrete 5.1 or 7.1 mix, but are intended to be heard.
An example I guess would be something like a plane approaching from behind. You hear the plane approaching in primarily the rear channels first, then it fills all channels, then as it leaves it is mostly in the front channels. If the rears were just dumped entirely, then you would be missing most of the audio from the approach and only really start hearing it at the point that it fills all speakers and then leaves the scene. I’ll stand to be corrected on that though. 😃
I believe that’s where the problem of downmixing the upmixed restorted track comes into play. The upmixed rear channels are being downmixed back into the front channels.