Have people heard of this?

Hi everyone,

I have been asked to submit a piece as a PDF and the print shop says that whenever they insert a CorelDraw or Microsoft produced piece they see lines within background images. I'm attaching a screenshot to show what she means. Apparently they appear when they import the PDF into Illustrator. The only way they can and add it to the background in illustrator.

Is this something anyone has seen before? I've been using CorelDraw for about 12 years and I don't think I've ever seen it happen or been told that anyone is having it happen to them.

Thanks I look forward to hearing other people's thoughts.

Phil