I have added this issue to the tracker:
Issue 2366 – Monitor placement and maximization is not remembered on monitors with different scaling than the primary one

I'm sending you an email with a development version of WinSCP to the address you have used to register on this forum.

I know there is another post about opening behavior after closing a maximized window with version 6.5, but my problem seems to be slightly different.
When I close a maximized WinSCP 6.5 main window on any of my 3 monitors (scaled 100%, 125%, 100%), it does not remember its current state.
When I reopen it, the main window looks like the attached screenshot.
I have reverted to version 6.3.7, which does not show this behavior and works perfectly. I am not looking for an immediate fix as 6.3.7 works fine for me, but wanted to report the issue.