Stroke converted to fill by itself in saving pdf

Hello

I encountered an issue, when saving illustrator file to print pdf. We put the plotter cutting line (spot color, named CutContour) into the design for the sticker cutting machine to understand, where to cut. It must be only one stroke (and no fill), because if it's expanded to compound path, then there are two outer lines and plotter will cut it twice almost at the same place and stickers will be ruined. But when lately, this spot stroke is somehow converted automatically to compund path, when saving to pdf . Not all the strokes but only this spot colored one and not in every file, but sometimes. I can't see that it is converted - in my computer (newest version of Illustrator)  it is OK, if I open this file again - spot colored stroke as it should be, but in my coworkers computer (previous version Illustrator) and printers computer it is converted to compund path. I tried this with new empty file and everything was OK, but with some client designs it happens. What could be the issue?