When I do the Dictation challenges (particularly Advanced), I often cannot distinguish the top note of the chord (harmony) from the melody itself. This is because they are played around the same register of the same instrument (piano) with no difference in dynamics.
I don’t think this is because my aural skills aren’t strong enough - I really believe it is impossible to get 100% on (say) today’s Advanced challenge without either knowing the tune beforehand, or with lucky guessing.
Please consider changing the Dictation test melody to a different instrument (e.g. clarinet sound) or always ensure the top note of the chord voicing does not go higher than the melody note.