Thanks for your report.
I have sent you an email with a debug version of WinSCP to the address you have used to register on this forum.
I've 3 Monitors connected to my Windows 10 PC. When opening WinSCP the window does not appear on the Position and with the size I've cloded the programm the last time.
Each Start of WinSCP i've to move the window to the upper left and resize the bottom to to the Taskbar.
I tried the position an size on all 3 Monitors, all handle this the same. The Size and position is not stored.
A long time ago this works like a charm, but since last year this bug looks like a feature.