It helps when you post like this that you include a comment that lets us reproduce your issue, and preferrably one without literal color/controlcodes. I finally figured out that you're not actually typing anything, but that that this must be a multi-line text in the editbox.
I'm guessing the difference is that the editbox using the font-linking built into windows, while the message area uses mIRC's own font-linking and text handlers. Not sure if the behavior happens also in win10, but in win7 if you set the font to "emoji one color", in the editbox it looks crazy with all letters you type getting smashed on top of each other - except for the SPACE. But the text looks normal once you press ENTER and the text shows in the #channel/@window area.
I also noticed something possibly related to your issue, with the unicode codepoints for the chess pieces, where they looked normal in the editbox, but in the channel window they looked badly, including the black queen looking like a silhouette of bart simpson. It turned out that mIRC's font linking includes Fixedsys Excelsior 3.01, and if a font matching that specific string is loaded, that the channel window area was showing a lot of characters by linking to Fixedsys Excelsior 3.01 even when the channel window was using a different font than that. When I unloaded that Fixedsys Excelsior version and switched to Fixedsys Excelsior NoLiga, the chess pieces showed as expected in the channel window area, because mIRC didn't find the font with the different 'name'.
