FDM release 4.9.242
We applied the CONCAT function to the CITIZEN.ISSUE_PLACE field.
The rule should concatenate the value of the CITIZEN.CITY field if it finds that ISSUE_PLACE is not null (GUI flag a null highlighted in yellow).
However, when the masking runs it always updates the field with the value of CITY, whether it finds ISSUE_PLACE to null or that it is valued.
This is evident from the results of the queries performed before and after the masking.
Release : 4.9.1
Component : Fast Data Masker
Keep Null option was not designed with work with CONCAT function. A product enhancement has been done to implement this feature.
This feature has been implemented in below build of FDM and can be applied with following article.