Search filter by Post title OR Meta Title Value [duplicate]

It seems to me that you have not changed the operator in the function attached to the get_meta_sql filter.
If $search_string is only fragment of the title, you should change post_title="%s" in code from this answer to:

$sql['where'] = sprintf(" AND ( %s OR %s ) ",
        // --- post title begins with the searched phrase --
        $wpdb->prepare( "{$wpdb->posts}.post_title LIKE '%s'", $title . '%' ),
        // -- OR post title contains the searched phrase --
        // $wpdb->prepare( "{$wpdb->posts}.post_title LIKE '%s'", '%'. $title . '%' ),
        mb_substr( $sql['where'], 5, mb_strlen( $sql['where'] ) )
);