File level trustees might be deleted when a file is modified, depending on how the application works with files it opens for writing. Some third-party applications record changes in a temporary file in order to save internal memory or as a safety net to prevent data loss due to a power failure, system crash, or human error. When a user saves the changes, the application deletes the original file, and saves the temporary file with same name as the original file. In response to the deletion instruction, the file system deletes the original file as well as any file level trustees set on the file. The file system is not application aware; that is, it does not track the ultimate intent of the applications that you might use.
For more information, see File-Level Trustees
in the OES 2018: File Systems Management Guide.