White Lines on JBIG2?

When I use jbig2enc to convert JBIG2 (with global) files into PNGs I'm
getting thin white stripes down the images in some places. When
extracting the pages with GhostScript the white line is not present.

Is this a bug? I would like to know how this can be fixed since I need
to decode the images without losing quality.

Thanks,
Alasdair