It's Windows 10.
Rather annoyingly, or luckily depending on your viewpoint, it's not showing that behaviour at the moment :) I started to notice it while I was setting up my new system, so often had it disconnected from mains power for extended periods of time. I've tried disconnecting it and leaving it for a few minutes just now but it's been OK, so far.
If it does happen again, I'll try the command and see if it solves the problem.