It's the handshake. The switch is reported to your source equipment that's it's a stereo only TV. That's because one of your TVs is stereo only. If the switch reported it as 5.1, then you wouldn't be able to hear anything on that set ever because there's no way to force stereo mode on some source equipment.
The issues is whether or not your source equipment can ignore the handshake "STEREO ONLY" report and output whatever sound format the user chooses. But if it's HDCP, then this isn't allowed unfortunately.