Select field not always saving depending on how the OPTIONS array is being built

For some reason my select field is not saving the value when I display the options this way,

'options' => array(
                array( 'value' => 'value1', 'label' => 'Label1' ),
                array( 'value' => 'value2', 'label' => 'Label2' ),
                array( 'value' => 'value3', 'label' => 'Label3' ),

And it is working fine when options are displayed this way

'options' => array(
             'value1'       => 'label1',
             'value1'       => 'label1',
             'value1'       => 'label1',
),

The problem is that I want to list the terms of a Taxonomy associated with another custom post type and I don't seems to be able to display the list of term and save the selected value using the 2 way of displaying the options.
Here is the actual code I use:

$optionsaddressloop = array();
    $terms = get_terms( array(
        'taxonomy' => 'partner_address_firm',
        'hide_empty' => false,
    ) );
    foreach ($terms as $term) {
      $optionsaddressloop[] = array(
          'value' => $term->slug,
          'label' => $term->slug,
          );
          }

$meta_boxes[] = array(
        'id'         => 'partner-admin-fields',
        'title'      => 'Partner Firm Admin Custom Fields',
        'post_types' => 'partner',
        'context'    => 'normal',
        'priority'   => 'high',

        'fields' => array(
          array(
              'name'  => 'Term for Address Loop',
              'id'    => $prefix . 'firm_term_address_loop',
              'type'  => 'select_advanced',
              'multiple'        => false,
              'placeholder'  => '-- select --',
              'options' => $optionsaddressloop,
              ),

          

        )
    );