Permalink Custom Page Template

You would probably be better served to not rely on the page-pagename.php method of applying custom templates to a page, and be more deliberate with assigning the template.

At the top of your page-manga.php file, include a template reference:

<?php
/*
Template Name: Manga Page
*/
?>

Now on your Manga page, find the Page Attributes metabox and select “Manga Page” from the Templates dropdown. Save/update the page.

This way no matter what permalinks settings you choose, WordPress explicitly knows which template to use for that page.

You can find more information on WordPress templates here: https://developer.wordpress.org/themes/template-files-section/page-template-files/

error code: 523