Hi,
Thank you for your interest in Syncfusion products.
I am afraid I am unable to reproduce the issue. I have created sample using your code and used your document to replace the content. Its works as expected. It removes the begin and end tags and its content.
Here is the sample for your reference:
http://files.syncfusion.com/samples/DocIO.windows/DocIO81331.zipPlease have a look into the sample and the output documents from the sample [..output\test.doc and output\dummy.doc]. Please try to reproduce the issue in it and send the same. Also please provide your Office version and Essential Studio version details. This would help us to investigate the issue further.
Best Regards,
Bhuvana