In brief: Microsoft has suffered through a steady stream of bugs in recent memory and the latest appears to be primarily targeting Windows 10 Pro users that upgraded from Windows 7 Ultimate or Windows 8.1 Pro via the free upgrade offer a few years back. Microsoft is aware of the issue and should have a fix ready to go soon.
Update (11/9): In a statement released to ZDNet, Microsoft indicates that they've solved the issue and that users should see an automatic fix to the activation bug within the next 24 hours:
"A limited number of customers experienced an activation issue that our engineers have now addressed. Affected customers will see resolution over the next 24 hours as the solution is applied automatically. In the meantime, they can continue to use Windows 10 Pro as usual."
Some Windows 10 Pro users are grappling with a mysterious bug that is deactivating licenses and downgrading them to Windows 10 Home. Feedback from Reddit seems to indicate that the issue is largely affecting users that upgraded to Windows 10 Pro from a Windows 7 Ultimate or Windows 8.1 Pro installation as part of the free upgrade program years ago.
Microsoft is aware of the issue and is sharing the following statement with concerned users:
Microsoft has just released an Emerging issue announcement about current activation issue related to Pro edition recently. This happens in Japan, Korea, American and many other countries. I am very sorry to inform you that there is a temporary issue with Microsoft's activation server at the moment and some customers might experience this issue where Windows is displayed as not activated.
Our engineers are working tirelessly to resolve this issue and it is expected to be corrected within one to two business days.
Aside from an annoying watermark, the issue doesn't seem to be causing any major issues at the moment. The best course of action, as Microsoft has suggested, is to simply wait until a fix is ready. At that time, I suspect the issue will automatically work itself out and everyone will be able to move on.
This is the latest in a string of bugs causing headaches for Microsoft. In October, the tech giant paused the rollout of Windows 10 October 2018 Update (version 1809) due to a file deletion bug. A security update earlier this year caused issues with USB and onboard devices.