For attachments uploaded before 1705 release, the "lastModified" date displaying in People Profile is not changed even we do changes for that record later.
However for attachments which were uploaded after 1705 release, the "lastModified" date is just showing the date when we changed the data.
SuccessFactors People Profile
This is an expected behavior.
Before 1705 release, the "lastModified" date was an importable field which can be modified manually with import, system will take that value in the import file to save as the "lastModified" date.
After 1705 release, we implemented an enhancement to the data-field "update", the "lastModified" date now become a system-generated value which you are not able to edit or change, it will just show the date whenever you imported the data.
Sample configuration in Succession Data Model:
<background-element id="careerDevelopmentPlan" type-id="61">
<label>Career Development Plan</label>
<data-field id="update" field-name="lastModified"max-length="999"max-file-size-KB="1000">
KBA , LOD-SF-EP-BGD , Background Elements , How To