… most likely because I disabled "show preview before edit box". The preview is displayed correctly "on first edit".
Category pages and File pages are some the only wikicontent rendering pages which 'append other stuff' in their View stage. For Categories these are: Pages, Subcategories or Files within that category (or a message that there are none of those). For File pages, it is a TOC, a 800px size 'original', the file history and metadata.
With LivePreview, we only parse the wikicontent, we don't add the 'view' stage elements. But when previewing existing page revisions (including apparently categories that are in use, but have no associated page ?) we apparently take some different steps, leading to different content being included. And once that content is present, it won't be removed....
This will be a difficult one to fix..
P.S. wonder what VE is planning here...