So please go back to the original problem. Post a log file showing the copy that "loses a few KBs".
Or if you want WinSCP to recognize that the file is not complete during synchronization, include file size into synchronization criteria:
https://winscp.net/eng/docs/scriptcommand_synchronize#criteria
Or if you want WinSCP to recognize that the file is not complete during synchronization, include file size into synchronization criteria:
https://winscp.net/eng/docs/scriptcommand_synchronize#criteria