I suggest you upgrade to 6.15 and test it then. It's only a bug if it exists in the latest version too - and I've tried what you said atleast 5 times in 6.15 XP pro with default font, and cannot reproduce this bug.
Check if SJIS/JIS conversion is on, or any of multibyte options in ALT+O > IRC > Messages - If they are, try disabling them and see if that helps, then report it.
Regards,
