Working memory (what most people think of as short-term memory) dysfunction is very common among ADHD folks and is likely part of the executive dysfunction that's the core of ADHD.

For example, a hyperactive type may be hyperactive as a means to stimulate executive function, thereby improving working memory as a result of increase in neurotransmitters (I forget which ones).

At a macro/diagnostic level, psychologists may consider it a symptom in their questions to help them differentiate. But everything I've read points to it being part of the executive dysfunction.