Impossible to have multiple aggregated fields if the backend provides the value.

Some backends like the Solr backend are able to directly set all search result values. This works for aggregated fields, too.
But as soon as you add second aggregated field, Search API's VIews integration breaks the search result by overwriting the values from one aggregated field with the values from the other one.

The issue is caused by the fact that Search API hardcoded sets the property path of an aggregated field to 'aggregated_field'. As long as the backend doesn't provide the data, the issue is hidden. But if it provides the data, this piece of code becomes active:

namespace Drupal\search_api\Plugin\views\query;

class SearchApiQuery extends QueryPluginBase {

   ...

  /**
   * Adds Search API result items to a view's result set.
   *
   * @param \Drupal\search_api\Item\ItemInterface[] $results
   *   The search results.
   * @param \Drupal\views\ViewExecutable $view
   *   The executed view.
   */
  protected function addResults(array $results, ViewExecutable $view) {
    ...
      // Gather any properties from the search results.
      foreach ($result->getFields(FALSE) as $field_id => $field) {
        if ($field->getValues()) {
          $path = $field->getCombinedPropertyPath();
          $values[$path] = $field->getValues();
        }
        ...
      }
    ...
    }
  }

...
}

For the database backend $field->getValues() doesn't return anything, but if the solr backend is configured to retrieve results this code fetches the values from the backend's result. But for any aggregated field the $path will be the same: 'aggregated_field'. As a consequence we only get the result of the last aggregated field because $values[$path] gets overwritten every time.

As a quick fix I tried to change the property paths in the index config to 'aggregated_field_1' and 'aggregated_field_2'. But this immediately caused errors in facets.