Okay, tracked it down.
The text appears correct until a Unicode string such as something in Kana (Japanese), Hangul (Korean), or similar appears in the window. The instant a string of that sort appears in the window, all the text in the window -- including old text -- immediately begins showing the problem documented in this thread.
The problem remains even if you close the channel window/exit the channel and rejoin. You literally have to restart the client to get it to go away (until someone spits out a Unicode string, rinse lather repeat).
안녕히 주무셨어요 was the string I was using to test.
I can put up some screenshots showing before and after if folks want. So again, this does appear to be an mIRC thing, and not a scripting thing or a "font thing". The OP probably just exited mIRC entirely, and it'll reappear for him the instant someone outputs something in Unicode.
