causes the problems. The frame member is the TabGroup's internal frame. You should not change the objects returned by AceGUI:Create directly, they all have a userdata member, that is safe to use.
So use the following line instead.
Normally the layout function of the tabgroups parent container widget would be doing this, but as you are taking the widget and placing it in your own frame you need to position it.
edit: One of the cleanups I'm going to do before RC1 is to find these cases where you need to dig into the widget and provide methods on the widget that allow you to do the same thing, in this case provide a :SetPoint method in all widgets that is passed onto the .frame member
The TabGroup in AceGUITest doesn't work (in the frame spawned by TestFrame()). Since this addon is currently the only documentation of AceGUI (other than the source), I would really like to see this fixed.