WP_Query, pre_get_posts and offset

Question

I can't get my head around, why offset seems to be forwarded from custom WP_Query in page template to the main query further down?!

From my functions.php

/**
 * ::: Filter 'Custom Post Type' Archive Main Query :::
 */
add_action( 'pre_get_posts', 'filter_cpt_posts' );

function filter_cpt_posts( $query ) {
    if ( ! is_admin() && $query->is_main_query() && ! is_post_type_archive( 'cpt' ) ) {
        return $query;
    } 
    else { // on is_post_type_archive( 'cpt' )
        $query->set( 'offset', '1' );
        return $query;
    }
}

From my archive-cpt.php:

<?php
    $loop_recent_post = new WP_Query( array(
        'post_type' => 'cpt',
        'posts_per_page' => 1, // number of posts to display
        'offset' => 0
    ) );

    if ( $loop_recent_post->have_posts() ) {
        while ( $loop_recent_post->have_posts() ) { 
            $loop_recent_post->the_post(); 
?>

<!-- HTML Code -->

<?php wp_reset_postdata(); ?>

and further down:

<?php 
    //query_posts( 'post_type=cpt&offset=1' ); //Get rid of `query_posts`
    if ( have_posts() ) { 
        while ( have_posts() ) {
            the_post();
?>

Posts start in main query with offset=0.

Why?

PS: Yes,

• I've read When to use WP_query(), query_posts() and pre_get_posts -- that's the origin of why I came to alter query via pre_get_posts,
• and wp_reset_query() doesn't make a difference after WP_Query, I've followed this rec.

Answer 1

It helps to write problems down.

After I haven't used pre_get_posts extensively, I focused too much on that area for the cause, but it was the if clause.

Changed to

if ( ! is_admin() && $query->is_main_query() && ! is_post_type_archive( 'cpt' ) ) {
    return $query;
} 
elseif ( ! is_admin() && $query->is_main_query() && is_post_type_archive( 'cpt' ) ) { // on is_post_type_archive( 'cpt' )
    $query->set( 'offset', '1' );
    return $query;
}
else {
    return $query;
}

Everything is working fine. I like pre_get_posts.