The BUTTONX_DOWN_MASK is still set in the MouseEvent when mouseReleased is
called. This only happens when the component is a lightweight component.
If you modify the test program below to use a Button instead of a JButton
the BUTTONX_DOWN_MASK will not be set when mouseReleased is called. This
is reproducible under both Windows2000 and Linux.

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class MaskTest extends Frame {
    public MaskTest() {
        super("Mask Test.");
        setSize(400, 400);
        //Button button = new Button("Check Me Out");
        JButton button = new JButton("Check Me Out");
        add(button);

        Toolkit.getDefaultToolkit().addAWTEventListener(new AWTEventListener() {
            public void eventDispatched(AWTEvent e) {
                if (e instanceof MouseEvent) {
                    MouseEvent ev = (MouseEvent)e;
                System.out.println("AWT EVENT LISTENER BUTTON1_DOWN_MASK? " +
                    ((ev.getModifiersEx() & InputEvent.BUTTON1_DOWN_MASK) ==
                        InputEvent.BUTTON1_DOWN_MASK));
                }
            }
        }, AWTEvent.MOUSE_EVENT_MASK);
            
        button.addMouseListener(new MouseAdapter() {
            public void mousePressed(MouseEvent e) {
                System.out.println("Mouse Pressed");
                System.out.println("-------------");
                System.out.println("BUTTON1_DOWN_MASK? " +
                    ((e.getModifiersEx() & InputEvent.BUTTON1_DOWN_MASK) ==
                        InputEvent.BUTTON1_DOWN_MASK));
                System.out.println("-------------");
            }

            public void mouseReleased(MouseEvent e) {
                System.out.println("Mouse Released");
                System.out.println("--------------");
                System.out.println("BUTTON1_DOWN_MASK? " +
                    ((e.getModifiersEx() & InputEvent.BUTTON1_DOWN_MASK) ==
                        InputEvent.BUTTON1_DOWN_MASK));
                System.out.println("--------------");
            }
        });
        setVisible(true);
    }

    public static void main(String args[]) {
        new MaskTest();
    }
}