In JRadioButton demo of SwingSet2Demo more than one radiobuttons can be selected at the same time.
After clicking on the other radiobuttons or mouse movement, the problem disappears.
There is some delay in changing the state of the button after clicking.
This issue can be seen easily in AquaLookAndFeel,
and less frequently in metalLookAndFeel, i.e. MetalLookAndFeel RadioButton take less time to recover its state.
 
This issue is not seen with the early access 1 build.