@d_lav

This seems to be related to this issue ✨ Views hooks not executed Fixed .
The data export display is not being rendered and therefore some hooks are not executed. The views data export module includes a hook for this: hook_views_data_export_row_alter
Here is an example:

function my_module_views_data_export_row_alter(&$row, \Drupal\views\ResultRow $result, \Drupal\views\ViewExecutable $view) {
  if ($view->id() == 'my_view') {
    $node = $result->_entity;  // Get the node object
    $data = getData($node);

    foreach ($row as $key => &$value) {
      if (str_contains($key, 'nothing')) {
        $value = setCustomValue($data, $key);
      }
    }
  }
}

I was able to make the migration work by using patch #52 and following a similar approach as described in #59.
Instead of migration_lookup, I used the entity_lookup plugin.

I added the following code to the migrations/d7_flagging.yml file:

entity_id:
    - plugin: entity_lookup
      entity_type: node
      value_key: nid
      bundle: 1
      bundle_key: status
      source: entity_id
    - plugin: skip_on_empty
      method: row

In this case, it looks up all nodes for a matching entity id.