I have ALT-O > IRC -> Messages configured as follows:

[x] Multibyte display
[x] Multibyte editbox
[x] UTF-8 display
[x] Font linking

And my channel window's Font dialogue is configured thusly:
Font: Arial
Script: Western
UTF-8: Display and encode

When I input the character ㅅ into that channel window's editbox, it appears in the editbox itself correctly. However, when I press enter, it echos into the window as an "unknown glyph symbol" (the [] box character).

Yet with the same mIRC configuration, most other Unicode characters, like ❀, display properly in both locations.

My understanding is that Font Linking works like this: for each Unicode character present on a line within your window (or within its editbox), mIRC finds a font that features a glyph for it, and displays that particular character with that particular font, regardless of what your "actual" font selection for that window may be. That way, you can see all characters properly even when using fonts like Fixedsys (255 characters) or Arial (WGL-4/652 characters) which may not include glyphs for them.

Well, that effect appears to be working for ㅅ when typing it into an editbox. But not for its display within windows on lines of chat. In fact, the only way I can make ㅅ appear properly in mIRC windows is by manually setting their fonts to Arial Unicode MS (apparently so that font linking doesn't need to be invoked, as Arial Unicode MS already supports ㅅ natively).

Any ideas? (BTW, packet sniffer confirms mIRC is sending ㅅ to the server with the correct UTF-8 encoding (E3h 85h 85h).)