Thanks for looking into it, I was able to reproduce this issue as well when using a different locale.
It appears that under a different locale, eg. Japanese, when mIRC sets the UTF-8 text in the titlebar, listbox, or other controls, Windows automatically converts the literal UTF-8 text into the code page of the locale (since mIRC is not a unicode application). This results in the UTF-8 text being modified. When mIRC later tries to use this text, it is no longer valid UTF-8. That is what appears to be happening in any case.
I am still looking into this issue to see if I can find an easy way to resolve it.
