Updating PO from PO LINK action on SO

Hello, 

I am trying to persist a custom field from a SO to a PO when the purchasing details are updated through PO LINK. Nothing seems to be erring, but the value is not being updated on the PO. The PO is no longer On Hold so the fields are not updateable through the UI. Could this be what the issue is?

        protected void SupplyPOLine_RowUpdated(PXCache cache, PXRowUpdatedEventArgs e, PXRowUpdated InvokeBaseHandler)
        {
            if (InvokeBaseHandler != null)
                InvokeBaseHandler(cache, e);
            var row = (SupplyPOLine)e.Row;

            // Retrieve the Purchase Order based on the OrderNbr
            POOrder poOrder = PXSelect<POOrder,
                Where<POOrder.orderNbr, Equal<Required<POOrder.orderNbr>>>>
                .Select(Base, row.OrderNbr);
            POOrderExt poOrderExt = poOrder.GetExtension<POOrderExt>();

            //Get the Sales Order Number from the SOLine
            PXCache soLineCache = Base.Caches[typeof(SOLine)];
            SOLine soLine = (SOLine)soLineCache.Current;

            string soNbr = soLine.OrderNbr;

            //Get the Project ID extention field from the SO
            SOOrder soOrder = PXSelect<SOOrder,
            Where<SOOrder.orderNbr, Equal<Required<SOOrder.orderNbr>>>>
            .Select(Base, soNbr);
            SOOrderExt soOrderExt = soOrder.GetExtension<SOOrderExt>();
            string projectID = soOrderExt.UsrProjectID;
            
            //Set the extension field on the PO
            poOrderExt.UsrProjectID = projectID;
            
            //Update the PO
            Base.Caches[typeof(POOrder)].Update(poOrder);
            
        }