When you assign a post to another author, it is the initial author who gets the mails when comments are made.
That should be the assigned author.

Another thing in the same mail :

At this point, both in the comment-mail as in the notification mail is supplied the link to
Edit/Delete a post
and to
Edit your subscriptions/notifications

In the mail the author gets, it is not necessary to put in the subscription line, but worse : in the mail the subscribers get, it is absolutely not desirable that someone (else then the author) sees the link to edit/delete a post...