This behavior is expected.
When DLP Discover classifies a file, the file is modified as the classification is written to it. As a result, the
Date Accessed and
Date Modified time stamps are modified. The original time stamps are
not restored. Other scan types that only analyze content within the file restore the original time stamps because the file isn't being modified.
NOTE: The
Date Accessed and
Date Modified time stamps can also be modified with other
Remediation Scans. These other Remediation Scans use rule reactions that modify files, such as Apply RM Policies and Remove Automatic Classification.