Strange WP_Query behavior

Hi,

I am experiencing very strange results using WP_Query. I want to display all posts in my left menu (custom theme). For the record, all the posts belong under category 1 or 4. My code is as follows, located in a file included as a template part leftmenu.php

wp_reset_query();
$args = array(
    'post_type' => 'post',
    'orderby' => 'ID',
    'order' => 'ASC'
    )
$my_query = new WP_Query($args)

Problem: In the resulting query, few posts from category 4 are missing.

If I modify the query as follows:

wp_reset_query();
$args = array(
    'post_type' => 'post',
    'category__in' => array(4),
    'orderby' => 'ID',
    'order' => 'ASC'
    )
$my_query = new WP_Query($args)

suddenly, the query is correctly returning all the previously hidden posts from category 4.

Let′s modify the query once again, to include both category 1 and 4:

wp_reset_query();
$args = array(
    'post_type' => 'post',
    'category__in' => array(1,4),
    'orderby' => 'ID',
    'order' => 'ASC'
    )
$my_query = new WP_Query($args)

and the result is the same as with the first query, some posts from category 4 are missing! I var_dumped $my_query to check.

Any hint what could be the possible reason for such a behavior? Am I missing something? If the first query doesn′t return some posts based on the post_type criteria, how can the second query return these posts when the criteria is even more strict (filtering based on category added)? And why the first query doesn′t return some posts?

Any help or hint is greatly appreciated, I already spent hours thinking this problem over and over and studying the Codex.