Thanks for posting your question here in the Community. I'm happy to lend a hand with the purchase orders in QuickBooks Online.
As an initial troubleshooting step, I recommend clearing the browser cache. The cache is in place to collect frequently used data. If the cache becomes too full it can potentially cause certain viewing and performance errors within the system. Clearing it will remove anything that's preventing the purchase order numbers from loading properly. Here's how to get this done in Chrome for example:
1. At the top right, click More (three vertical dots). 2. Click More tools and then Clear browsing data. 3. At the top, choose a time range. To delete everything, select All time. 4. Check the boxes next to "Cookies and other site data" and "Cached images and files." 5. Click Clear data.