Custom field added to Gutenberg block not picked up by $attributes

Hi there
I'm using MB for the first time but did some digging in its capabilities so far I like it a lot!

I have the following problem:
I created a custom field "ploegen". It's a select that gets data via the REST API from an external source:

<?php
if ( class_exists( 'RWMB_Field' ) ) {
    class RWMB_Ploeg_Field extends RWMB_Field {
      public static function html( $meta, $field ) {
      
        $club_key = rwmb_meta( 'club-key', ['object_type' => 'setting'], 'PSD Integrate' );
        $api_key = rwmb_meta( 'api-key', ['object_type' => 'setting'], 'PSD Integrate' );
        $api_secret = rwmb_meta( 'api-secret' , ['object_type' => 'setting'], 'PSD Integrate' );

        $curl = curl_init();

        curl_setopt_array($curl, array(
          CURLOPT_URL => "https://clubapi.prosoccerdata.com/teams/all",
          CURLOPT_RETURNTRANSFER => true,
          CURLOPT_ENCODING => "",
          CURLOPT_MAXREDIRS => 10,
          CURLOPT_TIMEOUT => 30,
          CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
          CURLOPT_CUSTOMREQUEST => "GET",
          CURLOPT_HTTPHEADER => array(
            "authorization: $api_secret",
            "x-api-club: $club_key",
            "x-api-key: $api_key"
          ),
        )
        );

      $response = curl_exec($curl);
      $err = curl_error($curl);

      curl_close($curl);

      if ($err) {
        echo "cURL Error #:" . $err;
      } else {
          $response = '{"content":' . $response . '}';
      
      }
      
      $data = json_decode($response);
      
      $dropdown = '<select name="Ploegkeuze" id="Ploegkeuze">';
      
      foreach ($data->content as $i=>$value) {
      
        $dropdown .= '<option value="' . $value->id . '">' . $value->id . ' ' . $value->name . '</option>';
        //echo $value->name . '</BR>';
      }
      $dropdown .= '</select>';
      
      
      //return sprintf($dropdown, $field['Ploeg'], $field['ploegid'], $meta);
      return sprintf($dropdown, $field['field_name'], $field['id'], $meta);

}
}
}
?>

I have added this field to my Gutenberg block:

add_filter( 'rwmb_meta_boxes', function( $meta_boxes ) {
    $meta_boxes[] = [
        'title'           => 'Wedstrijdkalender',
        'id'              => 'wedstrijdkalender-inhoud',
        'description'     => 'Dit blok geeft de komende 10 wedstrijden weer voor de geselecteerde ploeg.',
        'type'            => 'block',
        'icon'            => 'fas fa-futbol',
        'category'        => 'layout',
        'context'         => 'side',
        //'render_template' => get_template_directory() . '/page-templates/kalender_block.php',
        'render_callback'   => 'toon_kalender_block',
        'enqueue_style'   => get_template_directory_uri() . '/blocks/hero/style.css',
        'supports' => [
            'align' => ['wide', 'full'],
            'customClassName' => true,
        ],

        // Block fields.
        'fields'          => [

            [
                'type' => 'ploeg',
                'id'   => 'ploegid',
                'name' => 'Ploeg',
            ],
            [
                'type' => 'text',
                'id'   => 'hans',
                'name' => 'Hanzo',
            ],
        ],
    ];
    return $meta_boxes;
} );

but in the callback function, I can't read the field value (needed to get more external data via REST API:
$ploegid = mb_get_block_field('ploegid');

Due to this, the curl query can't be executed an no data is retrieved.

I hope anyone here can shine some light on this issue...