User-defined (Additional) Fields are missing from Purchase Orders screen
A refresh problem can sometimes occur in the Purchase Orders module whenever a manually-created PO doesn't immediately display the User-Defined (Additional) Fields. If this occurs, usually saving the PO, exiting the PO screen, and reopening the PO screen corrects it.
There exists an issue currently in Programming for this refresh problem. That said, though, it should be very rare that PO's are manually created. So, this problem should rarely be encountered. Most Purchase Orders should be generated via MRP.