title: WooCommerce not saving the SKU value
body: <p>I note that in Woocommerce, in my list of products, some products do not show an SKU number. When I edit the product, and go under Inventory, the SKU is blank. When I type in the correct SKU and update the product and then go back under Inventory, the SKU is blank again. I.e., the SKU is not getting saved. Any suggestions why this behavior is happening and any way to fix it?</p>
  
  <p>Thank you.</p>