WP_Query add extra JOINs after calling get_posts() with category__in in query arguments

[elancerteam]

I'm working with a custom query in WordPress to retrieve posts of a specific type and category. Here's the setup:

<?php
$args = [
    'post_type'      => 'post',
    'post_status'    => 'publish',
    'posts_per_page' => -1
];

$args['category__in'] = array_map('intval', $selected_categories);

$articles_query = new WP_Query($args);

echo '<pre>';
// Dumping SQL query before calling get_posts()
var_dump($articles_query->request);

$posts = $articles_query->get_posts();

// Dumping SQL query after calling get_posts()
var_dump($articles_query->request);
echo '</pre>';

The $selected_categories array contains IDs of categories I want to filter by. My expectation was that the SQL query generated by WP_Query would be consistent. However, I noticed a difference in the SQL query output before and after calling get_posts().

When calling get_posts() only the posts with the first category returns.

SQL Query before get_posts():

SELECT SQL_CALC_FOUND_ROWS wp_posts.ID
FROM wp_posts
LEFT JOIN wp_term_relationships ON (wp_posts.ID = wp_term_relationships.object_id)
WHERE 1=1
  AND (wp_term_relationships.term_taxonomy_id IN (1,17))
  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, 10

SQL Query after get_posts():

SELECT SQL_CALC_FOUND_ROWS wp_posts.ID
FROM wp_posts
LEFT JOIN wp_term_relationships ON (wp_posts.ID = wp_term_relationships.object_id)
LEFT JOIN wp_term_relationships AS tt1 ON (wp_posts.ID = tt1.object_id)
LEFT JOIN wp_term_relationships AS tt2 ON (wp_posts.ID = tt2.object_id)
WHERE 1=1
  AND (wp_term_relationships.term_taxonomy_id IN (1,17)
  AND tt1.term_taxonomy_id IN (1)
  AND tt2.term_taxonomy_id IN (1))
  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, 10

Why is WP_Query adding additional LEFT JOIN clauses (with aliases tt1 and tt2) after calling get_posts()? I only expect it to filter by the categories provided in $selected_categories. I suspect it might be related to how category__in is handled, but I’m not sure why these extra joins are introduced.

I have disabled all plugins and I don't use any pre_get_posts filter or anything.