Here is the code I gathered off the internet so far:

$args = array(
    'post_status' => 'publish',
    'posts_per_page' => '5000',
    'post_type'  => 'post',
    'meta_query' => array(
        'relation' => 'AND',
        'is_part_of_series' => array(
            'key'     => 'epic-series-select-series',
            'value' => $series_id,
            'compare' => '=',
        ), 
        'chapter_order' => array(
            'key'     => 'epic-series-chapter-number',
            'compare' => 'EXISTS',
        )
    ),
    'orderby' => array(
        'chapter_order' => 'ASC',
    ),
);

but for some reason, this gets the correct 3 posts plus some other posts – which I have no idea why are present. I checked the meta table in MySQL and only the correct 3 posts have the meta key with the value I am querying. Apparently, I am doing something wrong in the query! Any ideas?

I tried asking ChatGPT for help but it did not.