I literally just had this happen a couple days ago. I dual boot with Linux Mint and I generally don’t go into windows except for specific use cases. So sometimes it can be a while between boots for windows on my system.
I had to use a windows only program, but it was a quick 5 minute thing. Finished my task, shut it down and I get the update prompt. Fine windows, update and shutdown. Of course it reboots, but it defaults into Mint and I’m confused for a minute why my PC is still on when I come back.