I’ve noticed this happening recently to me and it coincides with the release of Avast that added Game mode. It usually happens when I alt-tab out of a game, and it doesn’t happen if I have Game mode disabled - which is my solution for the time being.
Edit: I have not noticed that the Game mode temporarily disables Windows update by design.
Edit 2: Regardless, Windows update stays disabled even after I exit the game. I suppose this is not intentional.