Are you still seeing this behavior or did this only happen twice? Is there another administrator in your organization that might have changed the sharing settings?
I didn't notice anything in the ArcGIS Online organizations I'm an admin in. If this is still occurring we would want to investigate it in more detail through Esri Technical Support.
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.
The map layers being dropped was release-related and was addressed shortly after the update. Other ArcGIS Online organizations were running into it but we didn't get too many reports.