@markvt: See https://winscp.net/tracker/2030
Before posting, please read how to report bug or request support effectively.
Bug reports without an attached log file are usually useless.
Can I sent you a mail with credentials for the FTP server?
Also, do you mean that you can upload other files with TLS 1.3? Can you post a log for that too?
Please attach a full session log file both for TLS 1.2 and TLS 1.3.
Can you post a log for that too?