Upon creating a third-party custom field in a Network instance, a user needs to configure a field-level Network Expression (NEX) Rule: vid__v to copy the record's Network Entity ID to this field. However, after setting up the desired number of Country Groups, it is not possible to update this value for all Country Groups. The user attempts to verify, copy, or type it. Browser cache is cleared. Apparently, the NEX Rule always saves for the first Country Group, but not for the others. Why is the user not able to save the NEX Rule for all Country Groups?
This is expected behavior. The newly created custom field in question is managed by the customer, and it is enabled in third party systems as well. This is the reason why the user cannot update the field.
The Country Groups are not updated because some of the Countries in the Groups are managed by a third- party system, and the custom field is configured as a Master Managed field in these systems. The user can either update the third-party system configuration, or in the Data Model page, remove the Countries managed by this third-party system from the Country Groups. Then, the NEX field is updated.
Veeva Network Online Help: Creating a Custom Field