Both those characters display fine for me in mIRC 6.35 under Windows XP and 7 in both the editbox and display window.
Font linking is a complex process and depends on a number of issues. For example, some badly designed fonts can declare that they contain a range of characters when they actually do not. If you are using such a font on your system (I have a few on mine), they might occasionally be chosen as the font to display a particular character when in fact they only contain the unknown glyph symbol for it. So, depending on the fonts you have installed you may or may not see the wrong character.
There are ways around this issue but they are not very practical and would slow the display down considerably and I am not sure how reliable they would be. That said, the font-linking routine has been optimized in the Unicode beta version of mIRC, although I am not sure if it will help in your case.
As for the Windows editbox, it is completely under Windows control and has its own font-linking method. In the case where a character does not display correctly in the editbox, that is probably due to the issue described above as well.
