java doesn't render two combining diacriticals in thai right

JavaBug.java

import java.awt.*;
import java.awt.image.BufferedImage;

/**
 */
public class JavaBug {

  public static void main(String[] args) {
    BufferedImage image = new BufferedImage(100, 100, BufferedImage.TYPE_INT_ARGB);
    Graphics2D graphics2D = image.createGraphics();
    StringBuilder text = new StringBuilder();
    text.appendCodePoint(3588);
    text.appendCodePoint(3640);  // combining diacritic mark, should combine with previous character (3588) ??
    text.appendCodePoint(3633);  // combining diacritic mark, should combine with previous character (3588) ??
    System.out.println("size should be one character long, like:" + graphics2D.getFontMetrics().charWidth(3588) + " but instead is:" + graphics2D.getFontMetrics().stringWidth(text.toString()));
    System.out.println("this should show one character, and no dash circles;" + text.toString());
  }
}
 // outputs "expected 8, got size 14"