Paradise, you said: "I just tried setting the split point to the bottom C (C1) and I kept the APC setting at fingered but there is a rhythm. In fact, you can even play chords up until the bottom E. So I'm wondering why there is a rhythm despite doing all you said."
It's not that important but if you have the split point at the bottom C on the keyboard I don't understand how you can play chords to the left of that point . . . Leave the APC on but don't start the rhythm for the test. The objective is to be sure all keys are in pitch across the keyboard. They should be with the split point at the left bottom.
In the normal centered position of the pitch bend wheel, all notes played are in pitch. If there is a failure of the variable resistance of the pitch bend assembly, the keyboard will think that the wheel is all the way up or down depending on the failure. In that case, the right hand voices will be shifted above or below the tuning center while the APC voices will stay in pitch. This sounds like what you are describing. Although this could be caused by the main board, I would first concentrate on the pitch bend assembly. Any competent technician can check the assembly with little effort.
Sorry Serge was not able to help.