WP_Query posts_per_page strange caching

Question

I want to make custom query loop with posts_per_page limit.

<?php
    $news = get_category_by_slug('news');

    $sidebar_related = new WP_Query([
            'category__not_in' => [$news->term_id],
            'post__not_in' => [$post->ID],
            'posts_per_page' => 4,
            'post_status' => 'publish'
    ]);

    if($sidebar_related->have_posts()) :
        while($sidebar_related->have_posts()) : 
            $sidebar_related->the_post();

            the_title();

        endwhile; 

        wp_reset_postdata();
    endif; 

This code returns me 10 (or another value) posts instead of 4.

$sidebar_related->request shows

"SELECT SQL_CALC_FOUND_ROWS wp_posts.ID FROM wp_posts WHERE 1=1 AND wp_posts.ID NOT IN (15190) AND ( wp_posts.ID NOT IN ( SELECT object_id FROM wp_term_relationships WHERE term_taxonomy_id IN (620) ) ) AND wp_posts.post_type = 'post' AND ((wp_posts.post_status = 'publish')) GROUP BY wp_posts.ID ORDER BY wp_posts.post_date DESC LIMIT 0, 4"

As you can see, there is LIMIT 4. But $sidebar_related->posts shows Array with 11 elements.

1. I've tried remove all plugins: nothing happens
2. I've tried to install my theme on empty blog: works correctly
3. I've tried to remove all caches (object cache, wp_query cache): nothing happens
4. I've tried to change category__not_in arg to category__in: works correctly
5. Add wp_reset_postdata(); before the loop: nothing happens

So I can't understand why it works this way. Any help will be appreciated

Answer 1

Ok, finally I've found the reason.

We must ignore sticky posts in WP_Query this way:

 $sidebar_related = new WP_Query([
            'category__not_in' => [$news->term_id],
            'post__not_in' => [$post->ID],
            'posts_per_page' => 4,
            'post_status' => 'publish',
            'ignore_sticky_posts' => '1'
    ]);

I hope it will help somebody.