A bug in UBTService_BlueprintBase makes it impossible to create "deactivation-only" BP implemented BT services.
1. In any project create a BP derived from BTService_BlueprintBase
2. Override only ReceiveDeactivation or ReceiveDeactivationAI
3. Try to use it
Notice those functions will never get called.
Expected: they do get called
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-25866 in the post.