Since v5.5.0 (Build 3839), editing with an external editor won't update file on remote server. I've tried with gVim and Notepad++ and the issue is the same. When updating a file with WinSCP internal editor, file is updated remotely.
I've tried protocols SFTP and SCP and it is the same problem.
When I enable session log, I can see that WinSCP is doing some operations when I'm saving my file with internal editor while I don't see any operations when I'm doing the same thing with an external editor.
All was ok with previous version of WinSCP.
My version of Windows is 8.1 64 bits.