Hi Den,
You may have the OTS LINK function activated. Please make sure that OTS LINK is OFF. Otherwise, turning on ACMP will automatically activate an OTS (One Touch Setting), depending on the currently loaded Style and the currently activated Main Variation. This will change the voices and volumes of the keyboard parts (Right1-3, Left).
By the way, to activate the Left split point, ACMP does not necessarily have to be switched on, but it is enough to switch the Left part on (with ACMP switched OFF).
Best regards,
Chris