Hi Peter
We noticed it twice a few days apart from each other. No other admin could have changed the sharing setting. In addition to this, we noticed a few other oddities after the latest AGOL release including map layer dropping out of a map, widgets in Ops Dash that lost reference to the map layers. Because we can't repro the behavior I am not sure tech support will be able to help. I was mostly looking to see if anyone else was having these type of issues.