Hi,
Sorry for not being there for a while.
I'm checking your backend. There are updates pending for Helix Ultimate and others. Is this intentional?
Can I take a backup of your site?
However, it turns out that, the issue occurs because the 404 page is linked via a menu item. Menu items in Joomla always return HTTP 200. In Joomla 5, 404 pages must be handled via the template (error.php). To get a proper 404 status, remove the menu item and use Helix Ultimate’s error.php instead. This will return the correct HTTP 404 status and resolve the SEO issue.
Best regards,