PDF exports of spot colors sperations are not correct

When I do a print to PDF of separations of a Color Spot file, the PDF is not correct and leads to misbehaviours of my Riso printer.

A little example of a drawing with 2 spot colors (Red and Blue), 2 vector shapes and 6 monochrome bitmaps in different shades of the Red and the Blue :

A print preview in CorelDraw of the Red printout is correct.

Only objects with Red are on that page :

After printing this to PDF (using CutePDF Writer), the PDF looks good:

But when looking closer to this PDF, the PDF is not correct.

One can see that:

1) all the bitmaps are transformed and sliced into hundred of bitmap lines:

2) Worse, all the Blue objects are also present on the Red page but a White color (not even transparent):

(These 2 screenshots are from a re-import of the PDF file into Corel Draw)

Those white objects and bitmaps are confusing the printer and leading to misbehaviour.

Is there a way produce better PDFs ?