Hi,
When I use "Add music" in Shimmer Desktop Layout, I can add a folder, but, in the Settings, there is no new folder added in the Library.
Is it normal ?

It's normal, the folders listed in the preferences are the simply the folders that can be checked automatically, folders are normally only added there if you add them from the button below (which only allow you to select a folder, not files, unlike when adding from the menu)
Personally when I add music, I just add it from "Add files or folders" in the menu, and selecting the few last modified folders, it's much faster than a full scan and it is enough when I know what I have added.

The preference dialog should be redesigned for many reasons, one of them is that I think it put too much importance into these folders, which are usually only one or a few. I have only one folder there, and I could remove it without affecting anything I do.