External Reference Changes Tracked in Transaction History?

Are additions and changes of the External Reference field in Install Base tracked in the Transaction Histories of Instances?

I see that the field is available in the historical table, but I cannot see a record of the change.

How is it that the query "select external_reference from csi_item_instances where instance_id = 4241895;" produces a value, but the query "select old_external_reference, new_external_reference from csi_item_instances_H where instance_id = 4241895;" produces nothing but rows of 'null' values for the instance?