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-18.104.22.168. If you are experiencing issues masking Excel files, please upgrade your FDM release to 22.214.171.124 or greater.
TDM Patches can be downloaded from https://support.broadcom.com/web/ecx/support-content-notification/-/external/content/release-announcements/Test-Data-Manager-TDM-Patches/16649
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.