That is odd. With a valid license that hasn't been revoked, it should not lose activation like this.
What was the source of your license (basically, the key)? Was it suspiciously cheap? If the license was from a dubious source, Microsoft will sometimes blacklist it later, so when the activation server is pinged after a Windows feature update, it will deactivate Windows. Then you need to enter a valid new key to get it activated properly again.