As discovered in T307251, VisualEditor after an edit modifies the Vector skin, adding last modified if needed, updating title and categories https://gerrit.wikimedia.org/g/mediawiki/extensions/VisualEditor/+/f247aea770e98a19636fba6ba03fe6e9055d977e/modules/ve-mw/init/targets/ve.init.mw.DesktopArticleTarget.js#1264
Going forward these may not be future proof and it may be better to provide Vector-specific handling.
More information
As demonstrated in Example skin, we're converting categories into menu data and moving away from a block of HTML:
- https://gerrit.wikimedia.org/g/mediawiki/skins/Example/+/78dbc3bb372ad69b49640a61e1046e233e211cea/templates/CategoryPortlet.mustache#2
- We're converting footer into menu data which means in future the following code may not work:
$( '#footer-info' ).prepend( $( '<li>' ).attr( 'id', 'footer-info-lastmod' ) );