I have subjects lines up to 900 characters and it still has the ${ACNID:XXX} at the end. I think around 960 to 990 is where the subject line has issues. The RFC for email messages has a defined limit around the 980 to 1000 range. There is also a per line character limit for email subjects.
Really the summary should only be a summary of the issue and not be very long. If you want to see the whole original message then you can do that in the individual alert view by selecting the 'Original Email' view/tab. Would it be helpful to attach the original email as an attachment to the email message that is sent to the assignee?