Hi Cody,
Thank you for posting your query.
We are afraid that we were not able to reproduce the mentioned issue. You can add html contents with the help of WParagraph.AppendHtml() method also. We have prepared a sample to illustrate the usage of both WSection.Body.InsertXHTML() and WParagraph.AppendHtml(), please have a look at the sample in the below link. If the issue with InsertXHTML() method still exists, could you please provide us the code snippet or sample application to reproduce the issue, so that we can sort out the cause of the issue and provide you a solution.
http://files.syncfusion.com/support/DocIO.Windows/F89530/F89530.zipPlease let us know if you have any further questions.
Regards,
Vijay