How to prevent queried posts from being added to cache?

The posts array result in WP_Query is mapped to get_post() (here and here) with:

$this->posts = array_map( 'get_post', $this->posts );

and that seems to be adding posts to the object cache even though the cache_results argument is set to false in WP_Query.

Within the get_post() function (here) we have:

$_post = WP_Post::get_instance( $post );

for the case when $post is neither a WP_Post or an object instance.

The WP_Post::get_instance method contains wp_cache_get() and wp_cache_set() calls.

So that could explain the behavior you see in your example.

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