How to export grayscale image as grayscale in PDF?

[feklee]

Steps:

1. Open the attached document in Xara Designer Pro X11. The included image has no color information. It is purely gray scale.
2. Export as PDF for commercial printing.
3. Extract the image from the PDF, e.g. using pdfimages: pdfimages grayscale.pdf img
4. Open the exported image in Xara or e.g. the Gimp.
5. With the color picker inspect the gray areas. They contain color information now. For example, what was #999999 in the original Xara document is now #8e8582.

The problem is that the colors do appear in print, as an undesired tint.

How do I properly export a PDF with all images as grayscale? In the actual document, only text has color.