Footer was introduced in {T6052} and its full width problems with full sidebars were fixed in {T6168}.
This regressed when the new project pane on the left was introduced, see screenshot in downstream https://phabricator.wikimedia.org/T94580
Quick'n'dirty hack would be to edit `phabricator-standard-page-footer` by changing `margin: 4px 16px;` to `margin: 4px 16px 4px 56px;` or such in the file `webroot/rsrc/css/application/base/standard-page-view.css`.