title: wc_get_products with two meta_query condition being ignored
body: <p>I'm tryng to do a wc_get_products with two meta_query conditions, but it doesn't work. It returns random results and I don't know why (When I refresh the page the products given change)</p>
  <p>The code that I'm using is:</p>
  <pre><code>$args = array(
              'post_type' =&gt; 'product',
              'post_status' =&gt; 'publish',
              'posts_per_page' =&gt; $itemsperpage,
              'paged' =&gt; $paged,
              'meta_query' =&gt; array(
              'relation' =&gt; 'AND',
              array(
                  'relation'  =&gt; 'AND',
                      array(
                      'key' =&gt; '_iop',
                      'value' =&gt; '2',
                      'compare' =&gt; '=',
                      ),
                  ),
              array(
                  'relation' =&gt; 'AND',
                      array(
                      'key' =&gt; '_iop',
                      'value' =&gt; '6',
                      'compare' =&gt; '=',
                      ),
                  ),
              ),
              'orderby'     =&gt; 'menu_order',
              'order'     =&gt; 'ASC'
          ); 
  
  $products = wc_get_products($args);
  
  </code></pre>
  <p>I want to show results which meta key _iop is 2 or 6, but doesn't work. ¿Where is the problem?</p>
  <p>Thanks!</p>