I’ve had emails in the last few hours on 2.5b2 with category in the subject name, so I don’t think it’s a bug specific to the beta. email_subject in settings is set to [%{site_name}] %{optional_pm}%{optional_cat}%{topic_title}.
Let’s rule out a configuration issue or plugin compatibility before we write it off as a Contribute > Bug
Was the second ) in the above a typo? or does a ) appear in your site setting?
Are there any plugins installed on the site?