As we detailed before, it was already known that anyone with a Windows 10 Pro, Education, or Enterprise license could try out Windows 10 S on any device, and then opt it into the Windows Insider program like usual. Oddly enough, though, Microsoft is now apparently pushing Windows 10 Pro builds to Windows 10 S Insiders.
According to the report, Brad Sams from Thurrott.com was running Windows 10 S on a Fast Ring Windows Insider Surface Pro, and after updating to the latest build his device was pushed to Windows 10 Pro, instead of Windows 10 S. While it seems like an issue or nuisance for those who want to stay loyal to the security and benefits of the Windows Store OS, it is not something that is earth-shattering. This is mainly due to the fact the Windows 10 Pro OS is basically the same as Windows 10 S but without app restrictions.
No one except Microsoft can be certain on what's the real issue, but this could likely be an activation or server side bug, Anyway, it's best not to enroll a Windows 10 S device in the Windows Insider program until this is resolved. For now, though, keep tuned for more in the days ahead.