She could not remember.

Hello. I wonder if what happened to one of the persons that live in me can be considered as amnesia. I was alone at home, and I remembered my dead grandfather. Then Jenny woke up and she was trying to remember when my grandfather died. But she realized that she could not remember anything relevant to my grandfather and she could not remember a single thing that had to do with my adolescence. So, is this some kind of amnesia met among people with some kind of dissociative disorder?

If she wasn't active when he was, she might not have had the memories about him. Not all parts share memories of the same things. Some only have memories for when they are active. If she was active when he was around though and does not remember, then it would be considered memory loss/amnesia.

It is called dissociative amnesia. The memories are accessible by some dissociated parts but not bu some others. When the person switches part, the memories become inaccessible causing amnesia.
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Daniel.