This is reproducible on Solaris 10 JDS system with mustang b57 onwards.

Run the attached testcase. It shows a Frame (Frame1) and a dialog. The dialog is toolkit modal and it blocks the Frame. Click the button 'Show Frame2' on the Dialog. Another Frame (Frame2) comes up. This frame is also blocked by the Dialog. Click the button 'Change Frame2's modality type'. This changes the Frame2 to be toolkit modality excluded. Now close the Frame2 by clicking the close button on the title bar of Frame2. Click the button 'Show Frame2' on Dialog again to show the Frame2 again. Now it can be seen that the task bar does not show an icon for Frame2. Click the button 'Hide Frame1' on Frame2. Now there are no icons shown on the task bar even though a Frame is visible.