Write automatic title at save_post (infinite loop)

There is a simple way, You need to use filter hook wp_insert_post_data
So the code should be like

add_filter( ‘wp_insert_post_data’, ‘set_post_title_with_field_value’ );

function set_post_title_with_field_value( $data )
{
    if ($data['post_type'] == 'sentence' ){
        $sentence_number = get_field('sentencia_no', $data['ID']);
        $data['post_title'] = $sentence_number;
    }
    return $data;
}

You need to add this code into theme’s functions.php.
So try the code and let me know the result.
Thanks