Embedded fonts in MKVs are replaced with Arial Unicode MS with fontconfig 2.10.92 (included in the latest Homebrew).
Detected file format: Matroska
[ass] FcFreeTypeQueryFace failed
AV: 00:00:04 / 00:22:52 (0%) A-V: 0.011 Cache: 94%
[ass] fontconfig: Selected font is not the requested one: 'Arial Unicode MS' != 'LTFinnegan Medium'
AV: 00:01:42 / 00:22:52 (7%) A-V: -0.002 Cache: 50%
[ass] fontconfig: Selected font is not the requested one: 'Arial Unicode MS' != 'Amienne'
[ass] fontconfig: Selected font is not the requested one: 'Arial Unicode MS' != 'Charme'
The culprit is fontconfig 2.10.92, as mplayer2 has the same problem: https://bbs.archlinux.org/viewtopic.php?pid=1260215
Switching back to fontconfig 2.10.91 fixes the problem.
Yep, this is a known fontconfig problem and mpv can't do anything about it. (Even if not, mpv doesn't use fontconfig directly, libass does.)
I guess we should leave this open until fontconfig is fixed, as many will hit this issue.
Bug report about a similar problem with Firefox: https://bugzilla.redhat.com/show_bug.cgi?id=946859
Contains links to the offending commit and the upstream Firefox bug report as well.
mplayer2 bug report, exactly the same issue: http://devel.mplayer2.org/ticket/230 (edit: it's the same issue, but unlike mplayer2, mpv's progress bar is not broken because it does not use characters for drawing the bar)
One user reported that he tried with fontconfig 126.96.36.199-g03216cc (which contains the fix), and that it doesn't fix the OSD issue. However, Firefox web fonts supposedly work again.
This is a bummer.
fontconfig bug report: https://bugs.freedesktop.org/show_bug.cgi?id=63922
Looks like it's definitely a fontconfig bug. The latest fontconfig release(s) don't work, and fontconfig git is broken too.
Pushed a commit that works around this issue with fontconfig git, but not with fontconfig 2.10.92 (AFAIK memory fonts are broken completely in that commit).
Should be fixed with fontconfig 2.10.93, which was released 2 days ago.
Summary: 2.10.92 is broken with anything that uses memory fonts, use 2.10.93 instead or any other version.