Here’s my meta query:

$taxonomy_term = 1494;
$args['meta_query'] = array(
        'relation' => 'OR',
        'mp_exists' => array(
            'key' => 'tdlrm_mp_'.$taxonomy_term,
            'type' => 'NUMERIC',
        ),
        'mp_not_exists' => array(
            'key' => 'tdlrm_mp_'.$taxonomy_term,
            'compare' => 'NOT EXISTS'
        )
    );
$args['orderby'] = array( 'meta_value_num' => 'ASC', 'title' => 'ASC' );

The posts where 'tdlrm_mp_'.$taxonomy key exists get placed first as expected. Hovewer, the rest do not get ordered by their title (actually, I can’t figure out what they are ordered by). What am I doing wrong?

This answer doesn’t help. I wrote my query according to this manual.