It should also be noted that this font also breaks in notepad. Type 123456789, once you get to 8, the font should completely change. This means it's not an mIRC specific issue and basically concludes it's the font's fault.
I've also noted that in 6.35 the font looks nothing like it should (all characters look like the 8 rather than the style of the other chars), so this is yet another issue.