Bernie, I think I understand what you want, but I'm not sure I have the answer. If you go to the APC Mode button I think you will find that the memory setting is on. Turn it off and see if that solves the problem. The voice will stop when you lift your left hand chord with memory off.

If you only want the bass note to sound, go to Part Setting and reduce the chord voice volume to zero. You should then get only the bass note for the chord you play with your left hand.