Changes only HCM Extract for Payroll payments extract always extracts full file

Summary:

We currently have a full file extract which sends the Check payment details to bank and now the requirement is to change it as changes only file . I have taken export of full file and import it again with new name and chosen changes only option. I have selected the Threading database item as Extract Employee person ID and marked few attributes(Person number, Check Number) as Key attribute.
The 1st run i ran it with all attributes to get the full file and subsequent runs submitted as changed attributes. Though there is no changes to data , i see every time full file is getting extracted.