Open a docx file in richtextbox, if numpages in headers do any subtraction, it can not display correctly. see the attached pictures for how to display in richtextbox and microsoft word 2016.
Hello Huan, it seems that the case is reproducible in some specific conditions. I would suggest opening a support ticket (https://www.telerik.com/blogs/how-to-submit-a-support-ticket ) and send us the problematic document there for further investigation.