How to reproduce :

• use the css file in attachment
• import it

  <link rel="stylesheet" href="{ametys:skinURL('css/print.css')}" media="print"/>

• preview with your browser the rendering for print
  > the rules are not applyed

next, import the same css file like this :

<link rel="stylesheet" href="{ametys:skinURL('css/print.css')}" media="print" data-donotminimize="true"/>

> the rendering for print is ok.