Hello Mario
After reading your post I went to the XD9 in my store, selected a voice (clarinet) and tried all the different harmony types while playing a style in the left hand. I do not hear anything strange at all. It played all the duet, trio, jazz, etc. harmony's just fine. Please, what is it you are detecting and what voices were you using when trying out the harmony.
George Kaye
Kaye's Music Scene
Reseda, California