Your currently selected font does not have the emoji. The font fallback from either Windows or mIRC is using hardcoded name fonts which don't exist on your system, the result you see in channel is probably the two surrogates which are used to form the emoji character in utf16.
So the result you see there is most likely normal, you would need to install the Windows fonts on your system and it should most likely solve the issue.
Here is the list of font that mIRC itself looks for when it can't find the character in your current font, which you would need to install:
-Microsoft Sans Serif
-Arial Unicode MS
-Tahoma
-Arial
-Lucida Sans Unicode
-Code2000
-Fixedsys Excelsior 3.01
For editboxes, which are handled entirely by Windows, there's some font fallback being done but arguably, over the years it's only doing a worse job than mIRC (and i'm not sure which font are used), so you would need to select a font which have the character you want in this case.
