Hi Karthik,
Thanks for your inquiry. I have tested the scenario with following code snippet and have not found the shared issue. I have attached the output Docx file with this post for your kind reference.
Document doc = new
Document();<o:p></o:p>
DocumentBuilder docBuilder = new DocumentBuilder(doc);
docBuilder.InsertHtml(@"Header");
doc.Save(MyDir + "Out.docx");
In case you are using an older version of Aspose.Words, I would suggest you please upgrade to the latest version (v13.6.0) from
here and let us know how it goes on your side. If the problem still remains, please attach your input documents here along with code for testing. I will investigate the issue on my side and provide you more information.