We’ve met the same issue.
There is the email from gmail. After converting it to *.mht by message.Save(stream, MhtSaveOptions.DefaultMhtml);
this is displayed correctly in a web browser, but in MS Word the header isn’t displayed.
This issue really critical for us, cause we need to use Word for open MHTs.
You can simply reproduce this by saving any email from gmail to msg or eml and after that convert it to mht and look at the result in MS Word.
Emails from another mail systems are converted to mht successfully.