I presume that’s the CSS that is the issue. It’s auto-added by WP/Gutenberg. It’s calculating viewport based on whole window, but part of the window is obscured by the ‘sider/header’. And as far as WP is concerned, there’s no sidebar in this instance.
Is there a fix that would let me use the ‘alignfull’ option but keep it to the main section and not go behind the sidebar?