That's helpful, thank you. Is there also a way to see the file transfer history?

To see list of files in a transfer, go to View > Queue > File List.
https://winscp.net/eng/docs/ui_queue#file_list

I confess I haven't actually tried the transfer, so I'll do a full test and see what happens. When I observed this appearance I aborted and added the files manually one at a time. In any case, as a user, I want to see the list of files I'm queueing, not a summary line that could represent any number. I often prepare a list of files to upload before pulling the trigger, and wouldn't want to lose information from that list just because I used multi-select to add files. A summary line undermines part of the purpose of the queue, in my opinion.

Now that I understand how it works, I suppose I can just keep it in mind and add files one at a time for max clarity. It's not a huge problem.

Ok, it might be misunderstanding. An entry in WinSCP queue represents a transfer, not a file. So indeed, if you select multiple files, just one entry is added.
https://winscp.net/eng/docs/ui_queue#information

But it represents just the selected files, not an entire folder. I've understood that you believe that the entire folder was uploaded. I do not think that's the case. Or do you believe otherwise? If you do, please post both recording and session log file of the same upload.

Video attached.

Thanks for your report.
Can you please do screen recording?

When I select multiple files in the local column to "upload in background", it only adds one entry to the queue that seems to represent the entire folder, not just the ones I selected.