Hey UE4Hobbyist,
I’m a bit confused as to why you’re spawning an instance of the controller in your level. Could you elaborate on your reasoning for this?
This same setup works fine if the controller is possessing a pawn. For example, if you create a Pawn, assign it an AI Controller, and then inside the Pawn Get AI Controller->Unpossess->Destroy (make sure the AI CON reference is assigned to Unpossess and Destroy) it will destroy it as expected.
Also, I recommend giving this a try in 4.15 to see if the same issue occurs.
Let me know if you have any further questions and let me know the results of the 4.15 test.
Have a great day