It appears that there was a regression in 4.25 regarding the AI blackboard. Spawning an AI controller will end up with a black board with SelfActor set to null.
The reason for this is that while AAIController::InitializeBlackboard sets the SelfActor to a valid pawn, when Blackboard->RegisterComponent() is called, which ends up calling UBlackboardComponent::InitializeBlackboard which resers the
blackboard memory.
Short term solution is just to set it again after calling RunBehaviorTree, such as
GetBlackboardComponent()->SetValue(GetBlackboardComponent()->GetKeyID(FBlackboard::KeySelf), GetPawn());