Order posts alphabetically with numbers but some of the posts has numbers in the title

I have this query:

 $args = array(  

    'post_type' => 'alerts',
    'post_status' => 'publish',
    'posts_per_page' => -1, 
    'orderby' => 'post_title', 
    'order' => 'ASC',
);

$alerts_loop = new WP_Query( $args ); 

The problem is that the order breaks in certain point. look at the imagen any ideas why it breaks. I have used:

'orderby' => 'title',
'suppress_filters' => true,
'ignore_sticky_posts' => true