I am unable to confirm this 'bug" on any of the systems that I have. I currently have systems with mIRC 6.16 & 6.17 running on Windows 98SE, ME, XP SP2, and a Linux system (Xandros)..a total of 8 possible OS/mIRC version combinations.

If you're running some combination that I haven't tested, specify your OS & mIRC version, and I'll be happy to see if I encounter the same problem using your configuration.