Dimension Pro: "Accordian 1" sound truncates long notes above B5

Odd problem. I wonder if anyone can help.

I am using the Accordian 1 sound in the 'Ethnic and World' collection that came with Dimension Pro. I'm using X1, upgraded to X1b.

Long notes with pitches above B5 end prematurely and abruptly (they don't go through the 'release' phase properly).

Why on earth should this be? And does anyone have a solution? I wondered about editing the .sfz file inc case there's something wrong with the segment that handles notes above B5, but I've never done that before and am slightly nervous about this.

HM