I doubt it was a simple file buffer (or report a bug).
major-mode you use better than me. So please if you
ask to add support for any particular
major-mode - write how you
install, configure and run yours.
Such complex buffers are called “special buffers”. A simple way to
restore them is to use
(wg-support 'inferior-emacs-lisp-mode 'ielm `((deserialize . ,(lambda (buffer vars) (ielm) (get-buffer "*ielm*")))))
To understand how this works - see Special buffers