James, I have submitted a feedback request, but here are screen shots that illustrate the problem. To reproduce, I simply copy some files to the S3 bucket noting that the mtime is correct. When I unmount and re-mount the drive, the mtimes have all been changed to S3 object creation/upload date.
Source files on left window, destination S3 folder on right. (Sorry for the extra-wide pics.)
Note that the timestamps are the same after I drag them over
I unmount/re-mount the S3 drive and the times change to S3 object creation date. Original timestamps are lost