After upgrading to v6, typescript complains about missing el property in smart select params. So I added a reference to smart select item as in the documentation.
Now when I open the smart select, it popups twice: first one as a popup and behind the popup as a page, and if I dismissed the popup it returns to the second one which is like a page.
I also tested this addition (adding the el property) in version 5.7.14 and I got the same behavior.
Note: I am using the react version, and firefox/chrome latest versions.