When the forum_replytouser is unset the mails are received from the noreplyaddress but the firstname and the lastname are not present in the "From:" mail field.
In previous versions you received a mail with:
From: FooFirstName FooLastName <email@example.com>
and now you receive a:
From: Do not reply to this email <firstname.lastname@example.org>
Now the users are required to open every mail notification to see the author. Maybe the behaviour in previous versions was more usable.