The app doesn’t actively search for broken links. The 404 page URL will appear in the Broken Links list once this page is visited. If no one has visited the 404 page yet, such links will not appear in the Broken Links section.
The active search is on our to-Do list and will be implemented in the future updates. Unfortunately, the exact timeframes can’t be provided. If you want to be notified once it's done, please contact us at support@nexusmedia-ua.com.