You're cross-referencing Machine Groups and the Machine View, they are different in function.
Machine Groups are static groups of machines you build and use to scan machine from. All changes to these groups are seen immediately in the tree in the left pane.
The Machine View (View > Machines) is a dynamic grouping of machines where machines are placed under the Machine Group name it was last scanned in from. This screen is populated when you perform a scan of a machine or group of machines. If machines are included in multiple Machine Group and\or have agents installed you will see the machine moving to the last 'container' the machine was scanned from. You should see a new entry in the Machine View when you scan machines from the newly renamed foobar1 Machine Group. The previous Machine Group name will remain until deleted from View > Machines. Lastly, the blank container in the Machine View are machine that were last scanned by agents.
@Charles: Got it. Thank you very much!
You're welcome, let us know if you need anything else.