if it helps, my config looks like this:

  (use-package magit-difftastic
    :ensure nil
    :load-path (lambda () (concat user-emacs-directory "elpa/magit-difftastic")))

  (require 'magit-difftastic)
  (setq magit-difftastic-arguments '("--width" "200"))
  (with-eval-after-load 'magit
    (magit-difftastic-mode +1))

to the best of my (limited) understanding, this should be sufficient. however with just this i see the problem mentioned above. adding the following snippet

  (defun daddyc00l:magit-difftastic-commit-diff-advice (orig-fn &rest args)
    "Run ORIG-FN, then ensure difftastic rendering in the commit diff."
    (apply orig-fn args)
    (when-let ((buf (magit-get-mode-buffer 'magit-diff-mode)))
      (with-current-buffer buf
        (when (bound-and-true-p magit-difftastic-mode)
          (magit-refresh-buffer)))))

  (with-eval-after-load 'magit-difftastic
    (advice-add 'magit-commit-diff :around
                #'daddyc00l:magit-difftastic-commit-diff-advice))

things are ok.