DTTG consistently fails to generate previews/thumbnails for webarchives. It seem to have no issues with other kinds of items. Works for PDFs, images, and Markdown files. Doesn’t work for bookmarks but doesn’t show the message either.
I can’t recall when exactly I noticed this but it was definitely already broken in the last version.
Desktop has no trouble generating thumbnails for webarchives and DTTG can see those so this in not a major issue.