I figured it out myself. There was a meta data field set for this specific customer. I found it in OrderGroup_PurchaseOrder.
For future visitors, updating database directly is dangerous and we advise against that. If you want to change an order status, this should be used:
var po = orderRepository.Load<IPurchaseOrder>(orderGroupId); po.OrderStatus = OrderStatus.Cancelled; //for example. orderRepository.Save(po);
which is much safer and future-proof.
How do I change a purchase order's status in the list of purchase orders in Commerce Manager by manipulating the database? I have found, and changed its status to Completed in the [OrderGroup] table. But that only takes effect in the POs detailed view.
Appreciate all help I can get.