This issue has been closed as 'Won't Fix' due to an extended period of time without updates. If this issue is important to you please let us know by posting on the AnswerHub or UDN, and Epic will re-open the ticket for further review.
Engine.AIControllerClassName is being used in APawn's CDO creation, and if game-side AIController class is indicated it may not be available yet, which results in APawn not having AIControllerClass set.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-17830 in the post.