Validation doesn't work

When the field is empty (invalid), you redirect to "/comic/create".
That is the browser send another request to the server.
So the $validation in the view is not the same instance as the Validation instance in the first request.

You saved the Validation instance into the flash message in the session.
You need to get it from the session.

See https://codeigniter4.github.io/CodeIgnit...ion-errors

ci-phpunit-test | CodeIgniter Testing Guide | CodeIgniter 3 to 4 Upgrade Helper