I don't have a technical answer for this question, but am wondering if the Vyvance was 'on board' while the testing was done.
We've run into similar problems with school being reluctant to grant extra time to 'such a bright boy who is doing so well.' It helped to bring in the social-emotional piece, and to point out the anxiety that DS was under trying so hard to do 'what he knows how to do' on tests, but just not being able to maintain focus and finger control to finish the job within the time allotted.
For better or for worse, reversing a gradeskip and moving to a school that puts a premium on discussion as a prime way to incite learning have had a very good effect on my DS14, doing 9th grade again this year. The public school was unworkable without the gradeskip, and way too much executive function challenge with it. I feel very fortunate with the current placement.
Hope that helps -
Peace
Grinity
