I found a discussion on this from 2014 that remained unsolved, but hoping something has been learned! Often, and now having reinstalled CCleaner in light of an update, it appears to be deleting my Windows local account. To confuse things further, it dos not do so consistently, I believe.
That was scary as can be the first time, as I couldn't log in to my own computer! I figured out that I could log into a Microsoft account (online) and then re-create a local account (which I do without a password, so that it automatically logs in). But I don't want this to happen.
Notice that it's a local account that's deleted, nothing involving connecting to the Internet. Thus it has nothing to do with cookies in any of my browsers. I rarely use IE or Edge, and Firefox always removes cookies on exit (except for one, The New York Times), but using Firefox itself does not delete my local account.
This behavior is new to me with a new computer and Windows 10. It did not happen in an older computer with Windows Vista. I presume has to be a checkbox in CCleaner under, perhaps System, but what?