What happens when you select the Tuner or an analog 2 channel source? If those play 2 channel in pure or direct mode as stereo, then there is a processing fault.
I have looked at your manual and there is nothing odd about this receiver.
If the tuner and analog sources work correctly, then there is a processor fault. Then the first thing to do is a reset back to factory defaults. You will have to go through the set up again. Tuner only has sound when I switch to a surround mode. I have done a factory reset.
If that does not solve it, then you receiver needs repair replacement. Has this receiver always done this, or something that developed?