Aaah, I had overlooked that option ! I must have ticked it in the early stages of discovering the program, not knowing what it was.
(Well, to be frank even after reading What is "Transfer each file individually on background"? that remains a bit cloudy in my eyes and I feel it ought to be overruled by the fresher request to not be asked for confirmations), but, well, case closed.

Thanks for solving the issue :)

You seem to have enabled option Transfer each file individually. That effectively creates a set of unrelated transfer batches running in parallel. The option Yes to All has validity for the batch only. It does not affect the other batches.

Thanks for your report. I was not able to reproduce your problem.
I have sent you an email with a debug version of WinSCP to address you have used to register on this forum.

Hello !

This is a bug report, I'm using the portable version of WinSCP 5.2.3 build 3436.

When I select a group of files in the windows explorer and drag and drop them into a remote folder in the WinSCP window, if these are files that are already present, I am prompted, most logically, with a dialogue asking if I want to overwrite the first file.

However, even if I choose "Yes to All", for every following file already present, I will be asked, again and again, if I want the remote file to be overwritten by the local file.

So, the "to All" option isn't taken into account.

I'm attaching a screenshot, for clarity.

Well, that's it. Thanks for the renewed efforts with that great program :)