It's been that way for a while as far as I can recall. I kinda thought it was by design, that the description was being pulled from the App Store and therefore you could not edit it. If it isn't supposed to be that way, then it's been broken for a long time now - like, years.
That hasn't been my experience. I was able to edit these prior, just like iOS apps that are also pulled from the App Store. I would check "Ensure that users view the description " and include company specific requirements or links to setup instructions.