Hello! One reason I can think of is that the pawn is still in sight after discarding the stimulus. If the pawn is successfully sensed on the next frame the perceived actor can’t be considered as forgotten.
The only place where OnTargetPerceptionForgotten is triggered is in UAIPerceptionComponent::ProcessStimuli(), so manually discarding a stimuli won’t broadcast the event, and if the pawn is still in sight a new stimuli will be created the next frame.