Converting QImage to Mono causes artifacts

When converting some QImages from QImage::Format_Indexed8 to QImage::Format_Mono using QImage::convertToFormat() with default dithering Qt::DiffuseDither the result contains artifacts. See the attached converted_artifacts.bmp. The artifacts are not visible with other dithering modes.

The attached example (qtbug67425.zip) can be used to reproduce the problem. The package contains also another image, which is created by removing features from the original image until the problem was not reproducible.