Perfect, thank you!

For all who follow, see https://winscp.net/eng/docs/rawconfig#dddisablemove and the above ticket.

Those applications probably do not support CF_PREFERREDDROPEFFECT, which WinSCP sets to "Copy", and they default to "Move". You can avoid that by preventing WinSCP from allowing "Move" altogether:
Issue 1884 – Optionally disable moving using drag&drop

(Windows 10 1809, WinSCP 5.14.2, 5.19.3, 5.19.4).

Dragging a file from remote into an application sometimes deletes the file after the application receives it.

When I dragged a file from remote into WinMerge, I noticed the remote file window refreshed, and the original file was gone, now only existing in %TEMP%. I tried again, with LibreOffice Writer, and the same thing happened. However, dragging the file into a new Notepad++ file, resulted in the file staying put on remote.

I've attached some configs, and a debug-2 log file.

This is a fresh install of Windows 10, I seem to recall encountering and defeating this issue before, but I can't recall how I did it.

If there is a workaround for this version of Windows (I can't yet upgrade), which version should I download?

Thanks