@jguijarro
Please note that Aspose.Words mimics the behavior of MS Word. If you perform the same scenario using MS Word, you will get the same output.
Instead of iframe, we suggest you please insert the bookmark as shown below in your HTML document.
<html>
<body>
<p><a name=iframe></a> </p>
</body>
</html>
You can insert this HTML into empty or any MS Word document, move the cursor to the bookmark and insert the desired content. E.g. you can load the content of ‘https://www.google.com/’ into Aspose.Words’ DOM (document) and insert it to desired location. Following code example shows how to achieve it. Hope this helps you.
Document doc = new Document(MyDir + "bookmark.htm");
DocumentBuilder builder = new DocumentBuilder(doc);
builder.MoveToBookmark("iframe");
Document google = new Document(@"https://www.google.com/");
builder.InsertDocument(google, ImportFormatMode.KeepSourceFormatting);
doc.Save(MyDir + "21.9.docx");