This issue has been seen on the Windows operating system.
The Refresh button does work if you add files to the directory, but it still does not update the Date Modified field for the files if they are changed.
To update the Date Modified field, navigate out of the directory and then back into it.
This issue has been traced back to a Java issue outside of the Xilinx design tools.
A Java fix has been requested, but information on when this will be implemented is unavailable.