I have looked at the post https://forum.stockmanagementlabs.com/d/537-purchase-orders for an answer as my issue is exactly the same. After the latest update, the Purchase order is created and a supplier is selected and all is saved and updated. The item list is blocked for use until "Multiple Suppliers" is chosen. Then, when adding an item, it does not show up until the page is refreshed.
I have cleared cache multiple times in the browser and hard reloaded the page with no difference. I updated WooCommerce and WordPress to the latest version with no success.