I can reproduce this bug in Xubuntu 9.10. I tried both Emacs 23 and mousepad with IPAMonaGothic, and the bug occurred.
emacs23 : 23.1+1-4ubuntu3.1 mousepad : 0.2.16-2ubuntu2 ttf-ipamonafont : release1.0.8-0ubuntu0ja3 (installed from Japanese Team Repository)
I can reproduce this bug in Xubuntu 9.10.
I tried both Emacs 23 and mousepad with IPAMonaGothic, and the bug occurred.
emacs23 : 23.1+1-4ubuntu3.1 0.8-0ubuntu0ja3 (installed from Japanese Team Repository)
mousepad : 0.2.16-2ubuntu2
ttf-ipamonafont : release1.