Hello,
Our theme uses its own custom 404 template, and that’s why Rank Math couldn’t track 404s at first. The Rank Math team already added a filter on your site, so now the 404 Monitor should be logging errors correctly.
To fix the blank layout of your 404 page, you just need to create a regular page in WordPress with the content you want (for example, a message, image, and button to return to the homepage) and then assign it in Theme Settings > Other > Custom 404 page.
This way, you’ll have a proper 404 design showing to visitors.
Best Regards,