As suggested in WP Query Args - Title or Meta Value I tried to query my Custom Content Type on Title and Meta Value alternatively.

It almost works, but only if I do not introduce a tax query (and I need it!!)

// general args
$args = array(
    'post_type' => 'linea',
    'posts_per_page' => 10,
);
// tax args
$args['tax_query'] = array( 'relation' => 'AND', 
                         array(
                             'taxonomy' => 'stagione_linea',
                             'field'    => 'slug',
                             'terms'    => 'summer'
                         ) );

// meta args
$args['meta_query'] = array(
                       array(
                        'key' => 'codice_linea',
                        'value'=> $term,
                        'compare' => 'LIKE',
                    )
);

// special arg here
$args['_meta_or_title'] = $term;

Filter function is the one I copied from the article above. I tried to echo out something to understand the problem.

add_action( 'pre_get_posts', 'searchByTitleOrMeta');
function searchByTitleOrMeta( $q ) {

    if( $title = $q->get( '_meta_or_title' ) )  {

      add_filter( 'get_meta_sql', function( $sql ) use ( $title )  {
        global $wpdb;
        // Only run once:
        static $nr = 0; 
        if( 0 != $nr++ ) return $sql;

        var_dump($sql['where'); die(); // DEBUG HERE!

        $sql['where'] = sprintf(
            " AND ( %s OR %s ) ",
            $wpdb->prepare( "{$wpdb->posts}.post_title like '%s' ", "%".$wpdb->esc_like($title)."%"),
            mb_substr( $sql['where'], 5, mb_strlen( $sql['where'] ) )
        );


        return $sql;
    });
  }
};

With no tax query (suppose $term = 'lorem')

["where"]=>
  string(395) " AND ( tm_posts.post_title like '{0acc5f40973efce47f8c42124c3734c97cc594f755a60224e0dd4cb02827c6b6}lorem{0acc5f40973efce47f8c42124c3734c97cc594f755a60224e0dd4cb02827c6b6}'  OR ( 
  ( tm_postmeta.meta_key = 'codice_linea' AND tm_postmeta.meta_value LIKE '{0acc5f40973efce47f8c42124c3734c97cc594f755a60224e0dd4cb02827c6b6}lorem{0acc5f40973efce47f8c42124c3734c97cc594f755a60224e0dd4cb02827c6b6}' )

If I set the tax query

["where"]=>
  string(177) " AND ( tm_posts.post_title like '{0406194437bac6b8187ce05f3bb4eecb83f0178715b7c8ca750a672ba53df8ac}lorem{0406194437bac6b8187ce05f3bb4eecb83f0178715b7c8ca750a672ba53df8ac}'  OR  ) "

I don't understand why...