title: WP Event Manager - custom filter doesn't show items
body: <p>I'm using the wp event manager plugin on WordPress and I'm trying to custom it. I created some custom fields in the back-end <code>_event_modalita_erogazione</code>. This field was compiled in every event created in the back-end.</p>
  <p>I added the code for a new filter in the functions.php file as the official guideline.</p>
  <pre><code>// ADD FILTER
  /**
   * Adding via filter or you can directly add in a template file
   */
  add_action( 'event_manager_event_filters_search_events_end', 'filter_by_modalita_erogazione_field' );
  function filter_by_modalita_erogazione_field() { ?&gt;
  
  &lt;div class=&quot;wpem-row&quot;&gt;
      &lt;div class=&quot;wpem-col&quot;&gt;
          &lt;div class=&quot;wpem-form-group&quot;&gt;
                  &lt;div class=&quot;search_event_types&quot;&gt;
                  &lt;label for=&quot;search_event_types&quot; class=&quot;wpem-form-label&quot;&gt;&lt;?php _e( 'Erogazione', 'event_manager' ); ?&gt;&lt;/label&gt;
                  &lt;select name=&quot;filter_by_erogazione&quot; class=&quot;event-manager-filter&quot;&gt;
                      &lt;option value=&quot;&quot;&gt;&lt;?php _e( 'Erogazione', 'event_manager' ); ?&gt;&lt;/option&gt;
                      &lt;option value=&quot;aula&quot;&gt;&lt;?php _e( 'Aula', 'event_manager' ); ?&gt;&lt;/option&gt;
                      &lt;option value=&quot;videoconferanza&quot;&gt;&lt;?php _e( 'Videoconferenza', 'event_manager' ); ?&gt;&lt;/option&gt;
                      &lt;option value=&quot;e-learning&quot;&gt;&lt;?php _e( 'E-learning', 'event_manager' ); ?&gt;&lt;/option&gt;
                  &lt;/select&gt;
              &lt;/div&gt;
          &lt;/div&gt;
      &lt;/div&gt;
      &lt;/div&gt;
  &lt;?php
  }
  
  /**
   * This code gets your posted field and modifies the event search query
   */
  add_filter( 'event_manager_get_listings', 'filter_by_state_field_query_args', 10, 2 );
  function filter_by_state_field_query_args( $query_args, $args ) {
      if ( isset( $_POST['form_data'] ) ) {
          parse_str( $_POST['form_data'], $form_data );
          // If this is set, we are filtering by modalita erogazione.
          if ( ! empty( $form_data['filter_by_erogazione'] ) ) {
              $event_modalita_erogazione = sanitize_text_field( $form_data['filter_by_erogazione'] );
              var_dump( $event_modalita_erogazione );
              $query_args['meta_query'][] = array(
                  'key'     =&gt; '_modalita_erogazione',
                  'value'   =&gt; $event_modalita_erogazione,
              );
          }
      }
      var_dump( $query_args );
      return $query_args;
  }
  </code></pre>
  <p>I added on the back-end the input field:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>Etichetta</th>
  <th>Tipo</th>
  <th>Placeholder / Opzioni</th>
  <th>Meta Key</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>Modalità di erogazione</td>
  <td>multiple selection</td>
  <td>aula videoconferenza e-learning</td>
  <td>_event_modalita_erogazione</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>and in the DB I have the association of the event with this data, 8 row in total
  | meta key| data |
  |:---- |:------:|
  | _event_modalita_erogazione  | a:2:{i:0;s:4:&quot;aula&quot;;i:1;s:15:&quot;videoconferenza&quot;;} |
  | _event_modalita_erogazione  | a:2:{i:0;s:4:&quot;aula&quot;;i:1;s:15:&quot;videoconferenza&quot;;} |
  | _event_modalita_erogazione  | a:2:{i:0;s:4:&quot;aula&quot;;i:1;s:15:&quot;videoconferenza&quot;;} |
  | _event_modalita_erogazione  | a:2:{i:0;s:4:&quot;aula&quot;;i:1;s:15:&quot;videoconferenza&quot;;} |
  | _event_modalita_erogazione  | a:2:{i:0;s:4:&quot;aula&quot;;i:1;s:15:&quot;videoconferenza&quot;;} |
  | _event_modalita_erogazione  | a:2:{i:0;s:4:&quot;aula&quot;;i:1;s:15:&quot;videoconferenza&quot;;} |
  | _event_modalita_erogazione  | a:2:{i:0;s:4:&quot;aula&quot;;i:1;s:15:&quot;videoconferenza&quot;;} |
  | _event_modalita_erogazione  | a:2:{i:0;s:4:&quot;aula&quot;;i:1;s:15:&quot;videoconferenza&quot;;} |</p>
  <p>but when I tried to use the filter and select &quot;Erogazione-&gt;Videoconferenza&quot; the page reported to me &quot;no elements found&quot;.</p>
  <p>The debug print is</p>
  <pre><code>string(15) &quot;videoconferanza&quot; array(13) { [&quot;post_type&quot;]=&gt; string(13) &quot;event_listing&quot; [&quot;post_status&quot;]=&gt; string(7) &quot;publish&quot; [&quot;ignore_sticky_posts&quot;]=&gt; int(1) [&quot;offset&quot;]=&gt; int(0) [&quot;posts_per_page&quot;]=&gt; int(10) [&quot;orderby&quot;]=&gt; string(10) &quot;meta_value&quot; [&quot;order&quot;]=&gt; string(3) &quot;ASC&quot; [&quot;tax_query&quot;]=&gt; array(0) { } [&quot;meta_query&quot;]=&gt; array(1) { [0]=&gt; array(2) { [&quot;key&quot;]=&gt; string(20) &quot;_modalita_erogazione&quot; [&quot;value&quot;]=&gt; string(15) &quot;videoconferanza&quot; } } [&quot;update_post_term_cache&quot;]=&gt; bool(false) [&quot;update_post_meta_cache&quot;]=&gt; bool(false) [&quot;cache_results&quot;]=&gt; bool(false) [&quot;fields&quot;]=&gt; string(3) &quot;all&quot; }
  </code></pre>