In the subscriptions module, the widget title does not pass the 'widget_title' filter as it is done for WordPress default widgets
I suggest that you modify the code near line 546 in subscriptions.php (the line refers to Jetpack v2.2.5).
echo $args['before_title'] . '<label for="subscribe-field">' . esc_attr( $instance['title'] ) . '</label>' . $args['after_title'] . "\n";
$title = apply_filters( 'widget_title', $title, $instance, $this->id_base ); // as done for default widgets echo $before_title . '<label for="subscribe-field">' . esc_attr( $title ) . '</label>' . $after_title . "\n";
I also took profit of the
extract($args); done at line 532