Hi,
Joomla version is 3.10.1 with Helix Ultimate v1.1.4 and screenshot is useless because the page, with Error 404 status, only shows header, footer and the message
Errore
Pagina non trovata
(Error Page not found)
instead of the complete error page with logo, home link...
In other words, as far as I can undestand, Joomla does what the new (wrong?) code lines tell it to do: it shows COM_SPPAGEBUILDER_ERROR_PAGE_NOT_FOUND message and sends 404 header.
$apps->enqueueMessage(Text::_('COM_SPPAGEBUILDER_ERROR_PAGE_NOT_FOUND'), 'error');
$apps->setHeader('status', '404', true)
Also, the redirect plugin doesn't work: 301 redirects set for changed URLs are not executed and new 404 errors are not collected, most probably because Joomla error is no longer raised, I guess.