Customer invoice - field Our Reference

This is what the online help says...

Reference:
The ID or name of the customer's contact, the default value will be retrieved through the contact fetching hierarchy. 

The  customer contact fetching hierarchy will not be valid in the following instances:
1. If a sales quotation/customer order is created from a business opportunity, the value defined in the Main Contact  field will be retrieved. If no main contact has been defined, the Reference field will be empty.
2. If a customer order is created from a sales quotation, the value defined in the Reference field on the sales quotation will be retrieved. If such a reference is not defined, the Reference field on the customer order will be empty.
3. If a return material authorization is created from a customer order, the value defined in the Reference field on the customer order will be retrieved. If such a reference is not defined, the Reference field on the return material authorization will be empty.

You can change the customer's contact by entering a new value in this field or by selecting one from the List of Values. The contact must be connected to the customer address.

Note: Customer contact fetching hierarchy is as follows:
1. Look at the Customer/Address/Order Info tab for the document address.
2. If it is empty, take the primary contact for the customer address defined on the Customer/Contact tab.
3. If that is empty, take the primary contact for the customer in general defined on the Customer/Contact tab.
4. If it is empty, then it takes the value from the Reference field on the Customer/Order/General tab.

Or, do you have a custom event in place populating that field with something else?