"Synchronize" command updates older files every time ...
Its not WinSCP causing the issue - Filezilla Server doesn't respond to "List" command accurately and this behavior is considered "acceptable" in that software:
https://forum.filezilla-project.org/viewtopic.php?t=8940
Filezilla Server responds to FTP "List" command with Date AND Time in timestamp for files with last modified time within the last month. For files older than a month FileZilla Server responds only with the date.
Looping through the FTP "List" returned and using FTP "MLSD" command on each file would get the correct Date and Time timestamp for all files overcoming the issue.
https://forum.filezilla-project.org/viewtopic.php?t=8940
Filezilla Server responds to FTP "List" command with Date AND Time in timestamp for files with last modified time within the last month. For files older than a month FileZilla Server responds only with the date.
Looping through the FTP "List" returned and using FTP "MLSD" command on each file would get the correct Date and Time timestamp for all files overcoming the issue.