The page_footer function was previously called on all pages which gave extensions access to the core.page_footer and core.page_footer_after events to override the page footer on all pages. As of the below ticket, those events are not run on app.php pages since page_footer, where those events are defined, is no longer called in the render method of controller_helper in the 3.3.x branch:

https://tracker.phpbb.com/browse/PHPBB3-16314