WordPress doesn’t select page template for parent pages

I’m developing a WP Theme and I’m having this issue that’s driving me crazy…

For every single (parent) page I create, WP chooses index.php as template. I’m aware of WP template hierarchy but it doesn’t matter if I use page.php, page-$slug.php or page-$id.php, I’ve also tried using:

<?php
  /*
  Template Name: My Template 
  */
?>

and assigning the page this template in the Editor.

I’ve also hooked, for debugging purposes, template_include as follows:

add_filter( 'template_include', 'check_template_selection', 99 );

function check_template_selection( $template ) {
    global $post;
    echo "Post ID: " . $post->ID . "<br/>";
    echo "Template File:" . $template . "<br/>";
    echo is_page() ? "YES" : "NO";
    return $template;
}

And for every page URL I check (for example: http://example.com/mypage) I’m getting:

Post ID:

Template File:/nfs/c11/h05/mnt/205383/domains/example.com/html/wp-content/themes/mytheme/index.php

NO

I’ve tried deleting the pages and creating them again, but it’s the same. The fact is it works fine for child pages. For example: http://example.com/mypage/mychildpage shows:

Post ID:589

Template File:/nfs/c11/h05/mnt/205383/domains/example.com/html/wp-content/themes/mytheme/page-mychildpage.php

YES

I’m using WP 4.4.2, ACF PRO 5.3.6.1, Custom Post Type UI 1.2.4 and Instagram Feed Pro Personal 2.0.4.1. No other plugins. Anyway, I tried unsuccessfully to desactivate them all. I’ve developed the theme from scratch and I’ve done dozens of themes before and I’ve never experienced this issue before…

This is my .htaccess and my permalinks are properly configured as /%postname%/, no category base.

# BEGIN WordPress

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

So please, any advice or hint on what am I missing here would be very apreciated. Thank you guys!

Edit:

After hours of deep search (Thank you so much John Blackbourn for your Query Monitor plugin) I found that my parent pages match the wrong rewrite rule, so now I guess it’s something about rewrite rules priority.

Wrong match for a parent page URL: http://example.com/about

Right match for a child page URL: http://example.com/about/about-child