Stupid Libreoffice Writer issue

I need the odd page with light grey lines for writing over, and print them a few at a time. It's simply a 24pt page filled with '_' in light grey.

I added a QR code (as picture, copy 'n' paste), and it looks fine; the QR code in black with the grey lines on a white background. When I try printing, or export to pdf, however, the grey lines vanish.

How do I sort that?