@sandy.wood Thank you for additional information. I have checked your document and noticed a problem with the table on the the 12th page. Actually it is not a table, but shape with text on the top of it. By the way, MS Word also cannot properly convert it HTML.
It looks like your document is a result of recognition from image or some other fixed page format. And the recognition software was not able to properly handle the mentioned table. Could you please let us know what was the original document format?
If your goal converting to HTML is to display (not edit) the document in browser, you can consider using FixedHtml format instead. In this case layout of your document will be preserved:
Document doc = new Document(@"C:\Temp\in.docx");