Custom query with search_api_facets hides other facet items when addCondition is added

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In case someone else facet the problem.
Solution is to register an event subscriber and modify Solr query according to documentation.

/**
 * Add tags to fq fields and facet.field fields to exclude each facet's
 * filtering constraints from influencing its own facet item's result counts,
 * to always keep the result counts of each facet item independent of that
 * facet's filtering.
 *
 * @see https://solr.apache.org/guide/7_6/faceting.html#tagging-and-excluding-filters
 */
class ProcessFacetFilterQuery implements EventSubscriberInterface {

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents(): array {
    return [
      Events::POST_CREATE_REQUEST => 'processFacets',
    ];
  }

  /**
   * Process facets fields.
   *
   * @param \Solarium\Core\Event\PostCreateRequest $event
   *   The event.
   */
  public function processFacets(PostCreateRequest $event): void {
    $request = $event->getRequest();
    $params = $request->getParams();

    foreach ($params['facet.field'] ?? [] as $facetField) {
      /*
       * Spotting an "ex=facet:" tag inside facet field value means
       * that facet is configured in OR mode. That works right.
       * Problem occurs, when that facet is also used in a query condition.
       * That makes it lose its values.
       * A modification to fq param should be applied in that case.
       */
      $fieldName = preg_match('/ex=facet:(.*)}/', $facetField, $matches) ? $matches[1] : NULL;
      if ($fieldName) {
        $this->modifyFilterQueryParam($params, $fieldName);
      }
    }

    $request->setParams($params);
  }

  /**
   * Modify fq filter param.
   *
   * @param array $params
   *   Request params.
   * @param string $facetField
   *   Facet field name.
   *
   *   If facet filter is being applied as a query condition,
   *   fq param should be modified to exclude facet filter
   *   from losing its values.
   *
   * @see https://solr.apache.org/guide/7_6/faceting.html#changing-the-output-key
   * @see \Drupal\search_api_solr\Plugin\search_api\backend\SearchApiSolrBackend::setFacets()
   */
  private function modifyFilterQueryParam(array &$params, string $facetField): void {
    foreach ($params['fq'] as $key => $field) {
      if (str_contains($field, $facetField)) {
        $params['fq'][$key] = "{!tag=facet:{$facetField}}" . $field;
      }
    }
  }

}