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In WP admin when looking at an order you got the following php warning (when display is enabled in wp-config):
Notice: Function is_internal_meta_key was called incorrectly. Generic add/update/get meta methods should not be used for internal meta data, including "_payment_method". Use getters and setters. Backtrace: require('wp-admin/post.php'), require('wp-admin/edit-form-advanced.php'), do_meta_boxes, WC_Meta_Box_Order_Data::output, do_action('woocommerce_admin_order_data_after_billing_address'), WP_Hook->do_action, WP_Hook->apply_filters, WC_Gateway_Purchase_Order->pofwc_display_purchase_order_meta, WC_Data->get_meta, WC_Data->is_internal_meta_key, wc_doing_it_wrong Please see Debugging in WordPress for more information. (This message was added in version 3.2.0.)I believe this is caused by using $order->get_meta(‘_payment_method’, true ) instead of $order->get_payment_method() on line 720 of wp-content/plugins/purchase-orders-for-woocommerce/class-purchase-order-gateway.php