Modified At metadata not being copied when migrating files from File System to SP Online

I am migrating files from file system to SharePoint Online and following Uploading files into SharePoint from user manual.

But, its not migrating the modified at value from the file system.

The file is copied to SharePoint online with the date it was copied to SharePoint but not when it was last modified on the file system.

I have tried all combination in the field value selection  - keep Original value and Modified At from available columns but nothing seems to work.

I will appreciate if some one can guide what I am missing. Thanks