Paginate WPQuery redirect to page 1 - php

Question

In my Wordpress template, I'm making a WP query in order to list all cities in specific regions (custom post type).
In some cases, regions have more than 3000 cities so the Query is too heavy and runs out of memory.
Therefore I'm using the paginate_links function in order to paginate each list when there are more than 500 results.
It works fine but the paginate links are somehow redirected to the main page.
Examples :
http://www.example.fr/my-region/page/2
redirect to
http://www.example.fr/my-region so I can never access to the paginated pages.
Here is my code (partial).
Do you see what may cause this problem?
<?php
$paged = ( get_query_var( 'paged' ) ) ? get_query_var( 'paged' ) : 1;
$posts = array(
'post_type' => 'commune',
'posts_per_page'=> 500,
'paged' => $paged,
'orderby' => 'title',
'order'=> 'ASC',
'meta_key' => 'region',
'meta_value' => $reg_region
);
$communes = new WP_Query($posts );
$nb_communes = $communes-> found_posts;
?>
<div class="col-8 main-content">
<?php while (have_posts()) : the_post(); ?>
<?php
echo "<ul class=\"list-group\">";
while ($communes->have_posts() ) { $communes->the_post();
?>
<li class="list-group-item"><?php echo(get_field("commune")); ?> (<?php echo(get_field("cp")); ?>) - <?php echo(get_field("civilite")); ?> <?php echo(get_field("prenom")); ?> <?php echo(get_field("nom")); ?> </li>
<?php
}
echo "</ul>";
?>
<?php endwhile; // end of the loop. ?>
<div class="pagination">
<?php
echo paginate_links( array(
'base' => str_replace( 999999999, '%#%', esc_url( get_pagenum_link( 999999999 ) ) ),
'total' => $communes->max_num_pages,
'current' => max( 1, get_query_var( 'paged' ) ),
'format' => '?paged=%#%',
'show_all' => false,
'type' => 'plain',
'end_size' => 500,
'mid_size' => 1,
'prev_next' => true,
'prev_text' => sprintf( '<i></i> %1$s', __( 'Précédent', 'text-domain' ) ),
'next_text' => sprintf( '%1$s <i></i>', __( 'Suivant', 'text-domain' ) ),
'add_args' => false,
'add_fragment' => '',
) );
?>
</div>
I do not know if it's related but in my functions.php file, I have this override so that my cystompost type skip the /$name_of_custom_post_type/ in URLs.
/**
* Remove the slug from published post permalinks. Only affect our custom post type, though.
*/
function gp_remove_cpt_slug( $post_link, $post, $leavename ) {
$custom_post_types = array('departement', 'region', 'commune');
if ( ! in_array( $post->post_type, $custom_post_types ) || 'publish' != $post->post_status ) {
return $post_link;
}
$post_link = str_replace( '/' . $post->post_type . '/', '/', $post_link );
return $post_link;
}
add_filter( 'post_type_link', 'gp_remove_cpt_slug', 10, 3 );
/**
* Have WordPress match postname to any of our public post types (post, page, race)
* All of our public post types can have /post-name/ as the slug, so they better be unique across all posts
* By default, core only accounts for posts and pages where the slug is /post-name/
*/
function gp_parse_request_trick( $query ) {
// Only noop the main query
if ( ! $query->is_main_query() )
return;
// Only noop our very specific rewrite rule match
if ( 2 != count( $query->query ) || ! isset( $query->query['page'] ) ) {
return;
}
// 'name' will be set if post permalinks are just post_name, otherwise the page rule will match
if ( ! empty( $query->query['name'] ) ) {
$query->set( 'post_type', array('post', 'departement', 'region', 'commune' ) );
}
}
add_action( 'pre_get_posts', 'gp_parse_request_trick' );