I think you may have the OTS button on. This is designed to change with the registrations buttons. Two things you can try:-

1) turn off the OTS button choose your style, and put the FREEZE button on.
2) Choose your Style and voice, and save to Registration 1. Make any further changes and save to Registration button 2, and so on. e.g. Piano in Right 1 saved to Regit 1,
Piano in Rt1 and Strings in Rt2 saved to button 2 and so on.

With the FREEZE button on this will enable you to keep the same style throughout your song whilst allowing variation to the voice section.

Hope this helps - Audrey