I think the Not! Them creature leaves a few people aware of it’s existence so they have fear to feed on. That’s a pretty common theme from entities we see in TMA, but what I think is more is the way the entity picks who can remember it strategically.

In the case of Rose Cooper, the entity lets her daughter remember her real identity. Her daughter has had a strained relationship with her Mother for years, and when she tells her Father that her Mother isn’t her Mother, her Father dismisses her by saying “This isn’t the way to handle your issues.” The daughter’s lack of relationship with her Mother makes her seem unreliable to person she needs to believe her the most…which only serves to elevate her fear and frustration in the situation.

In the case of Sasha James, the entity lets Melanie remember her real identity. Melanie only met Sasha once, and she was much less close to her than the rest of the Institute employees. That makes her word about Sasha being a different person seem much less credible to Jon than if Tim or Martin came to him and said something was wrong…which only serves to elevate Melanie’s fear and frustration in the situation because, from her perspective, it genuinely seems like Jon is randomly gaslighting her about Sasha’s whereabouts and identity.

The creature doesn’t just pick someone to remember it, it picks a person who wasn’t close enough to the person it replaced to be believed. And that’s so much more scary of a concept than if it just picked at random.