Problem with backup

Notepad++ is realy great and usefull tool, which I use in many cases.
And one of most usages is as its name says -- I use it as notepad.
In my case, I usually have an unsaved session, consisting of unsaved files.

But an unfortunate situation arose (I lost the text I worked on for 4 days):
I closed an unsaved file in session and its backup-file disappeared from the backup folder.

Description in screenshots for clarity:

For example, I have a session with an unsaved file ("new 1" in example).
-- its backup in the backup folder.

The problem is that when I close "new 1" in Notepad++ session, backup of "new 1" in backup folder will be lost too.

If I close Notepad++ (application) (with some unsaved files like "new 2", "new 3" opened) backup files will keep saved in backup folder.

Behaviour varies.

I would like to make it to:
(Automatically) save backup files (in the backup folder) even of unsaved files that was closed in the session.
This will probably increase the number and size of backup-files in the backup folder, so this might be optional.

Start by reproducing the reported sequence with an unsaved session: close one unsaved file, then inspect the backup folder, and compare it with closing the application while other unsaved files remain open. Trace the session and backup handling entry points responsible for file closure and shutdown. Done means closed unsaved files retain backups, with any optional behavior reflected in the issue's requested setting.