Yep the notification was originally triggered yesterday after the DO uplift. I've cleared it twice yesterday and once today after with two reboots as well. It seems to come back every Action Centre refresh.
That must be a bug in Action Center (which doesn't surprise me as it and the rest of the Windows 10 shell are still riddled with them; Action Center is often confused here, just from the Windows Update notifications). There's no way Opus could go back in time, to before a reboot, and re-insert a notification in the past which you had already cleared.