Is Dysport FDA approved for the forehead?

That is correct that Dysport is FDA approved in those areas. We use Dysport off label all over the face ie. neckbands, masseter, crows feet etc. Dysport does tend to spread more so that may be what you mean by migration. Because it spreads more the injector may decide to use a different dose, or even dilute it differently.
It is very common in the medical field to use medications off label, this is true for many of the toxins and fillers that are used in the medical aesthetic field.

Neuromodulators (such as Botox, Dysport, Xeomin, and Jeuveau) and soft tissue fillers like Belotero can improve the appearance of the forehead by reducing lines, wrinkles and folds. Deeper lines and wrinkles need a combination approach with Botox, fillers, lasers, and microneedling/PRP. I recommend getting a formal evaluation with a cosmetic dermatologist to develop a treatment plan that works for you. Best, Dr. Emer

It is common for neuromodulators such as Botox and Dysport to be used "off label" to treat varying concerns of the face and neck. Each neuromodulator on the market is designed a bit differently so it is not surprising that you have some different experiences when using Botox versus Dysport. If you have one that you prefer, simply stick to it. Many injectors offer more than one neuromodulator product and will adjust to your preference.