AI perception by default detects all pawns without further setup required. For all other actors to be perceived, you need to register them as stimuli source. To do this, add a AIPerceptionStimuliSource component to an actor that you want your AI to perceive. Add all senses you want an AI Perception Component to react to and enable “Auto Register as Source” on this component. This component also allows you to register and unregister an actor at runtime.
The pawn class does not have this component and will be detected by default. if you wanted to disable automatic detection of pawns without that component, add the following lines to your DefaultGame.ini:
[/Script/AIModule.AISense_Sight]
bAutoRegisterAllPawnsAsSources=false