It's not a bug, it's a feature:
https://winscp.net/eng/docs/resume#automatic

Just one more information :

The bug above is present when the option "Options" / "preference" / "endurance" / "Enable transfer resume/Transfer to temporary filename for" : choice "File above : 10KB" is selected.

If the option : "Options" / "preference" / "endurance" / "Enable transfer resume/Transfer to temporary filename for" : choice "All files" is selected, the bug is ALWAYS present, for all files.

If the option : "Options" / "preference" / "endurance" / "Enable transfer resume/Transfer to temporary filename for" : choice "Disable" is selected, the bug is NEVER present.

And my version of winscp : 5.9.1 (Build 6885)

Hello,

I have a probleme, for exemple, I upload and open a "CSV" file (owner : root, group : psacln, size of file : 468 KB) usind winscp (sftp with an account root) and notepad++.

I make any modification in my file.

And I save the file to my server.

When I verify the file's autorization of my "CSV", the owner is "root", ok, but the group is "root", it is'nt normal...

Thank you for your help.

Johnny