That sounds like something my kid might do. He often used his ability to memorize to compensate when he could not write fast enough or neatly enough.
It is hard for people to understand this "inconsistent performance" thing and it is one of the reasons why my son never got OT for his dysgraphia. By the time he got to a point where it was harder for him to compensate, it was too late for therapy. Because he wasn't in OT, people found it even harder to believe that he was not just lazy. There were people in his scout group that I am sure thought I was lying about his disability, but there was one person who understood and saw both the giftedness and the disability. That person made a big difference and kept me from just totally losing it. Because she was the mom of gifted sons and also a special ed teacher she recognized the signs.