Looks like something's gone wrong on Microsoft's side with regard to the Insider program for Windows 10 S. Users are now reporting that Windows 10 S users enrolled in the Fast Ring of the OS are receiving Windows 10 Pro instead, with the latest update.
While there was previously some confusion as to what was going on, Microsoft's Brandon LeBlanc has now issued a statement clarifying that this is simply a labeling issue. The build users have received is still Windows 10 S, it's just showing the wrong name for an as of yet undetermined reason.
We're looking into this but even though it shows as Windows 10 Pro, it runs as Windows 10 S. Shouldn't impact you if you want to stay on Windows 10 S. https://t.co/Vq3Rvk4L0P— Brandon LeBlanc (@brandonleblanc) January 4, 2018
Other users are also reporting that as a result of this issue, those looking to actually upgrade to Windows 10 Pro are unable to do so, with the upgrade option not accessible. This is noteworthy due to Microsoft's recent extension of the free upgrade offer from Windows 10 S to Pro for users on the Surface Laptop.