I'm guessing that you set the font for the wrong window. It could also be that there are some exceptions which will not cause this bug to manifest itself on certain platforms different from mine (I use a very standard XP SP2 installation)..
Steps to reproduce this bug:
1) Install a clean mIRC 6.17
2) Set nickname to "testABC"
3) Connect to any server (I used a QuakeNet server)
4) Type "/debug @debug"
5) Type "/query testABC"
6) Right-click on this query window's switchbar button, and click on "Font.."
7) In this menu, set UTF-8: to "Display and encode", and press OK
8) Now type "/notice testABC,testABC test" in any window
9) In the @debug window, you will see "-> servername NOTICE testABC :test"
Works for me every time..
