QuizAccessDeniedSubscriber breaks system.site settings for 404 repsonses

I have added custom functionality to handle the 404 pages.
Please review.

I checked 49. Following steps are applied to check issue.
Created a custom 404 page and set it as a 404 destination page from configurations.
Checked by malformed URL, it is showing the desired page.
Installed quiz module.
But after this, not getting same page on the malformed URL, because of the issue as mentioned in description.
Applied MR as a patch and got correct specified 404 page.
Moving it to RTBC now.

Should have test coverage

Fixed the pipeline, added test coverage, and did a review.

Automatically closed - issue fixed for 2 weeks with no activity.

The function on404(ExceptionEvent $event) states that it "Handles 404 errors specifically for quiz-related routes."
Or, no test is done on routes. In result dblogs are plenty of 'Caught 404 exception.' thrown by $this->getLogger('quiz_subscriber')->warning('Caught 404 exception.'); on every 404. Please see the function on403() route tests and apply accordingly to on404().