Hello and welcome - hopefully, someone here with a similar experience will be able to comment.
This is certainly unusual, given that you have no inflammation and a definitive diagnosis of Graves'!
The only other thing I can think of would be to ask whether your diet includes a lot of high iodine foods (such as the seaweed wraps that come with sushi) or if you've recently had any type of diagnostic testing using contrast dye.
You might also ask your doctor if a low-iodine diet might be helpful as you prepare to re-do the test. Most endocrinologists do NOT require this for a scan, but mine did. You can find more info on this at the ThyCa.org
website, as the low-iodine diet is standard practice for thyroid cancer patients who undergo RAI. (It's the same treatment that Graves' patients receive, but with a much larger dose).
Take care - and keep us posted!