All of the masked Excel files got with FDM (4.10.9) are broken:
Release : 4.10
Component : Fast Data Masker
This has been resolved in FastDataMasker-220.127.116.11. If you are experiencing issues masking Excel files, please upgrade your FDM release to 18.104.22.168 or greater.
If the patch you are looking for is not available for download, please open a support case, and let support know what patch you need.