Not necessarily. The only thing you should plug into the Phones Jack is a pair of Headphones. This is a TRS (Tip-Ring-Sleeve) connection. If the Phones are not plugged in fully this can be the cause of just one side working.

Do you mean when you plug the MOXF into an external sound system? Make sure you are using two standard TS (Tip-Sleeve) cables when plugging in to your sound system. You don’t tell us anything about you “sound system” so there is nothing we can tell you about whether that should work or not. In fact what you say is you “...plug in an external sound system...” we are sure you mean you plug the keyboard into an external sound system... but then you tell us nothing about the sound system.