Stereo output JP15 Issue?

Just purchased first Music Man guitar. I’m concerned there may be an issue and if anyone has an idea please let me know. When plugged in the mono output, sounds fantastic even when putting the toggle switch to the Piezo up position. However when I put a cable in the Stereo output (and the toggle switch in the up position), no sound at all. I do keep the original cable in the Mono output. As soon as I take the cable out of the Stereo output, all three positions work as they should. I did buy it used so just not sure if I’m doing something wrong. Any thoughts would be greatly appreciated. Thank you.