*SPOILER ALERT* In the movie Sookshmadarshini something doesn’t make sense to me

In the movie at the end the climax turns out to be portraying Ammachi as the mastermind behind everything. But if she was part of the plan all along why didn’t she do anything and act all confused and frightened when Priya(Nazriya) tried to confront her? I mean a simple yes I’m okay or some kind of act to confirm that she has Alzheimer’s would’ve put Nazriyas doubts to bed. Felt like that part wasn’t making any sense in the movie. Thoughts?