Printing does not fully respect grayscale colorspace with Qt::Dense4Pattern

The following code from KTextEditor-5.31.0:

paint.fillRect(0, lineHeight(),
    range->shiftX() - xStart, lineHeight() * (range->viewLineCount() - 1),
    QBrush(config()->wordWrapMarkerColor(), Qt::Dense4Pattern)
);

prints a colored pattern albeit selecting Grayscale in the printer dialog.

The non-grayscale color of the pattern in PDF output file is different from time to time - which suggests that the issue might be caused by an uninitialized C/C++ variable.