I doubt the loss of your IPv6 connectivity and the Windows update have anything to do with each other. This comes to you from a machine with Windows 10 Professional Version 1511 and the latest Windows updates - it has perfect IPv6 connectivity. I just fired up a virtual machine running Windows 10 Professional Version 1607 (the Anniversary Update) with the latest Windows updates installed and that, too, has perfect IPv6 connectivity.
The increase in speed suggests that you FTTC connection resynchronised. It is possible you are now connected to a gateway that doesn't provide IPv6 connectivity, especially if you are on an ISP where IPv6 is being tested, rather than being a fully supported part of the package.