You have to know the container of the button. At least you have to
find a frame or a dialog in which this button resides. Then you can
find a button by several criteria and also you can implement its own
ComponentChooser with custom condition. For example,

  JFrameOperator contOper = new JFrameOperator();
  JButtonOperator buttonOper = new JButtonOperator(contOper, index);

finds index-th button in given container. Or

  JButtonOperator buttonOper = new JButtonOperator(contOper, new
ComponentChooser() {
            public boolean checkComponent(Component comp) {
                String tooltip = ((JButton)comp).getToolTipText();
                if(tooltip!=null) {
                    return tooltip.equals("My Tooltip");
                } else {
                    return false;
                }
            }
            
            public String getDescription() {
                return "tooltip equals to My Tooltip";
            }
        }
        );

finds a button with tooltip equals to "My Tooltip" string.

So, I suppose this is not a bug. Jemmy can find components by several
ways. Please, next time ask similar questions at the
users@jemmy.netbeans.org mailing list. There is a community of users
which is willing to help you.