In mIRC 6.33 I was typing a bunch of Japanese characters in the edit box, and then did a tab complete on a nickname and it turns to ? marks... Didn't use to happen. (Friend that's still on 6.31 doesn't have the problem)
My Options in 6.33: (IRC -> Messages)
I tried all combinations of enabling and disabling both "Multibyte" items (four combos, figure it out.. not hard)
I also have both Font linking and UTF-* display enabled, with proper Encode and Decode enabled in the Font option area.
No I'm not turing on ShitJis it's a bunch of balls, so w/e, not the issue here.
Friends Options in 6.31: All six are checked
Simple way to recreate this error:
1) Use the Japanese IME (if you're any knowledgable about it you would have it set up, if not google is your friend.)
Set to Japanese mode, set to Keyboard, type in some Japanese (I used 'baka')
2) Alternativly you can probably paste some in there from a properly unicode encoded website, I didn't test this since I can enter it in my own.
3) Hit enter then hit space. (allows it to complete the enter of text, if you've used IME you understand it)
4) Exit Japanese Keyboard entry (so you can type in English now)
5) Type the first two characters of your name, or type #, and then hit TAB.. auto complete works fine, for channels and nicks, However... the Japanese will now be a set of ? marks.
My guess is that when you created the auto-resized editbox, something with tab complete ate balls and so now the unicode somehow gets massacred. Beyond this simple test.. I don't know enough on how to explain what's going on in the background. That's Khaled's job

Hope you can figure it out and get a fix to this and other errors (I'm going to go post others now), thanks.
EDIT:: I reverted back to 6.32 (for other errors I can't deal with) and it does the same thing... so now I'm unsure what caused it.