Usage of MB Group and custom Field

Hello,
i need to let a user type a long url in a field and then upon saving using a service to short url and provide tracking. I was trying to use the filter rwmb_{$field_id}_value but they are not working for me when using groups. So i went ahead and create a custom field in the following way :

class RWMB_Partner_Field extends RWMB_Text_Field {

	/**
	 * Get meta values to save.
	 * Save terms in custom field in form of comma-separated IDs, no more by setting post terms.
	 *
	 * @param mixed $new     The submitted meta value.
	 * @param mixed $old     The existing meta value.
	 * @param int   $post_id The post ID.
	 * @param array $field   The field parameters.
	 *
	 * @return string
	 */
	public static function value( $new, $old, $post_id, $field ) {
		error_log('new value to parse : '.$new);
		$calc = self::shortUrl($new);
		return $calc;
	}
}

when used outside of a group this function get called without any issue. But when using it in a group (actually 2 level of nesting) this never get called.
I notice that also if i define a save method it doesn't get called either, while a get_attributes will get called.

here is my structure used :

add_filter( 'rwmb_meta_boxes', 'sa_release_mb' );

function sa_release_mb(){
	$prefix = 'sa';
	$mb[] = array(

		'id'         => 'release',
		'title'      => __( 'Release Infos', 'sa_mb' ),
		'post_types' => array( 'post'),
		'context'    => 'normal',
		'priority'   => 'high',
		'fields' => array(
			array(
				'name'  => __( 'Release', 'sa_mb' ),
				'id'    => $prefix . 'release',
				'type'  => 'group',
				'clone' => true,
				'sort_clone'  => true,
				'collapsible' => true,
				'fields'=> array(
					array(
						'name'  => 'Model',
						'id'    => $prefix.'model',
						'type'  => 'text',
						'size'  => 60,
						'columns'=> 12

					),
					array(
						'name'  => 'Colorway',
						'id'    => $prefix.'cw',
						'type'  => 'text',
						'size'  => 60,
						'columns'=> 12
					),
					array(
						'name'  => 'SKU',
						'id'    => $prefix.'sku',
						'type'  => 'text',
						'columns'=> 12
					),
					array(
						'name'  => __( 'Main Retailer', 'sa_mb' ),
						'id'    => $prefix . 'main-retailer',
						'type'  => 'group',
						'clone' => false,
						'collapsible' => true,
						'group_title'   => 'Main Retailer',
						'fields'=> array(
							array(
								'name'  => 'Vendor',
								'id'    => $prefix.'main-vendor',
								'type'  => 'post',
								'post_type' => 'partner',
								'field_type'=> 'select_advanced',
								'columns'   => 6
							),
							array(
								'name'  => 'Link',
								'id'    => $prefix.'main-link',
								'type'  => 'partner',
								'columns'=> 6,
								'size'  => 45

							),
							array(
								'name'  => 'Price',
								'id'    => $prefix.'main-price',
								'type'  => 'number',
								'columns'   => 4
							),
							array(
								'name'  => 'Currency',
								'id'    => $prefix.'main-currency',
								'type'  => 'select',
								'placeholder'   => 'Choose Currency',
								'options'   => array(
									'EUR'   => '€',
									'DOL'   => '$',
									'UKP'   => '£'
								),
								'columns'   => 8
							),
							array(
								'name'  => 'Date',
								'id'    => $prefix.'main-date',
								'type'  => 'date',
								'columns'   => 12
							),
							array(
								'name'  => 'Time',
								'id'    => $prefix.'main-time',
								'type'  => 'time',
								'js_options' => array(
									'timeFormat'    => 'HH:mm z',
									'timezoneList'  => array(
										array('value' => '+0100', 'label' => 'CET'),
										array('value' => '+0000', 'label' => 'GMT')
									)
								),
								'columns'   => 12
							)
						)
					),

					array(
						'name'  => __( 'Other Retailers', 'sa_mb' ),
						'id'    => $prefix . 'other-retailer',
						'type'  => 'group',
						'clone' => true,
						'collapsible' => true,
						//'group_title'   => array('field' => $prefix.'vendor'),
						'fields'=> array(
							array(
								'name'  => 'Vendor',
								'id'    => $prefix.'vendor',
								'type'  => 'post',
								'post_type' => 'partner',
								'field_type'=> 'select_advanced',
								'columns'   => 6
							),
							array(
								'name'  => 'Link',
								'id'    => $prefix.'link',
								'type'  => 'partner',
								'columns'=> 6,
								'size'  => 45

							),
							array(
								'name'  => 'Price',
								'id'    => $prefix.'price',
								'type'  => 'number',
								'columns'   => 4
							),
							array(
								'name'  => 'Currency',
								'id'    => $prefix.'currency',
								'type'  => 'select',
								'placeholder'   => 'Choose Currency',
								'options'   => array(
									'EUR'   => '€',
									'DOL'   => '$',
									'UKP'   => '£'
								),
								'columns'   => 8
							),
							array(
								'name'  => 'Date',
								'id'    => $prefix.'date',
								'type'  => 'date',
								'columns'   => 12
							),
							array(
								'name'  => 'Time',
								'id'    => $prefix.'time',
								'type'  => 'time',
								'js_options' => array(
									'timeFormat'    => 'HH:mm z',
									'timezoneList'  => array(
										array('value' => '+0100', 'label' => 'CET'),
										array('value' => '+0000', 'label' => 'GMT')
									)
								),
								'columns'   => 12
							)
						)
					)
				)
			)
		)
	);
	return $mb;
}

Hoping to receive some help or a workarround as i need to get in production this code pretty soon and i'm running out of option.