Yes, I have seen somebody with ADHD fixate on something and the memory issues remain present while actively engaging the subject of fixation. The difference between the compulsive behaviors versus the obsessive behaviors only comes down to motivation and personal interests. This is where the dopamine cycle is impaired either way while in some cases motivation is sufficient to push through the limiting effects of supremely low serotonin. Consider the flip case where a ADHD person absolutely cannot be compelled to do something important because the lack of motivation is a greater driver of behavior than cognitive reasoning can compensate for.

There is a greater than 60% overlap in symptoms between ADHD and Autism Spectrum Disorder. The distinction is as clear as ADHD is more directly tied to memory issues and ASD is not while ASD is more directly tied to lost processing of social framing and ADHD is not.