There are quite a few items that have this issue, however, after checking 3 of them which have an incorrect balance in the P.Order field of the item states bar, they all have something in common.
They all have a Returned Goods to Supplier record for the same qty that is in the item status bar.
The Returned Goods to Supplier record is set to reduce quantities “received only” and the purchase orders are all un oked and open.
At some point, for all 3 purchase orders, there was a second goods receipt after the returned goods record which set the Purchase Order status to completely received and then a purchase invoice for the total amount.