This request is being tracked already:
Issue 1003 – Preserve owning group when uploading file to temporary file name
You can vote for it there.

Hmm, I understand.

after the original file is deleted and temporary file is renamed to the original file name, you lose all attributes of the original file (such as owner and group) except for basic permissions

Wouldn't it make sense to do the following?

• read attributes before deleting the file
  
• set the attributes to the temporary file (warn if this does not work)
  
• rename the temporary file to the original file name
  

A warning message would also be very helpful if attributes such as owner and group cannot be transferred, because a production system can become faulty due to incorrect attributes.

What do you think?

That's a side effect of the resume support. If you do not like it, turn the resume support off. It's documented here:
https://winscp.net/eng/docs/resume#automatic

WinSCP Version 6.1.1

How to reproduce:

• Remote server has a file, owner=root and group=www-data
  
• Log in as root
  
• Upload a new version of this file from local to remote. Ensure that the resume support is enabled which is normally the case for larger files. My file has 101KB.
  
• After the new file is uploaded, file group has changed to root
  

So, the file group of the updated file is not kept here! This is a serious bug.