Check this one, more about.....favicon
This is what happened..
I don't know if it matters, but I am using the Custom Static Pages plugin. It seemed harsh to move the widgets to drafts, but was devastating to have all the widgets erased.
That might be a bug. It's definitely not static pages doing it. I don't use that plugin and it happens to me too.
This is definitely a bug. Please go ahead and submit it as an issue on Github.