Yes, this problem is still here... I've not been able to find any information as to a workaround. The click problem that leciel mentions happens, as well as the same thing when you select text with multibye characters.
mIRC supposedly has a "Multibyte character" mode that "will improve the display, and mark/copy behaviour, of text containing multi-byte characters in mIRC windows" (under options -> irc -> messages) but it doesn't seem to matter whether I have that mode checked or not... it still has the same error.
Here's a screenshot (png, 15k). The top is the unselected, properly displayed text. The bottom is the selected, broken text.
I haven't had a chance to check this in 6.02; that's what I'll do next. But regardless, the bug is there in 6.03, and needs to be fixed.