When creating the part attribute, instead of copying and pasting the value from the feature attribute, use the "link to expression" option and choose the feature's attribute expression.
NX 11 changed things up a bit. There are two "radio" buttons above "Value", choose the "expression formula" option. The "Value" text entry will change to a drop down entry, choose the "formula" option from the drop down and then pick the feature's attribute expression.