If you have a page with slug /slug and in that page you add a custom pagination or query string ?page=MyCustomPage and then reload the browser, Worpdress will rewrite the url to /slug/MyCustomPage
If you want to prevent this from happening you should add the following filter in your theme’s functions.php file
add_filter( 'redirect_canonical', function ( $redirect_url ) {
if ( is_page_template( 'page-with-custom-paging.php' ) ) {
$redirect_url = false;
}
return $redirect_url;
} );