How to set font to get rid of boxes in Asian Phonetic Guide dialog

I’m using LibreOffice (on Ubuntu 14.04). In the Asian Phonetic Guide box, some characters show up as boxes, like this:


If I uncheck Use sytem font for user interface (in Tools > Options > LibreOffice > View), I get fewer boxes:


This is odd, because the system fonts I have set support the character replaced with a box as far as I can tell. (I use KDE, but this happens as well if I set OOO_FORCE_DESKTOP to gnome).

I assume I need to somehow tell libreoffice to use a different font than whatever it’s using here, but I’ve got not idea what that font is or how to change it, and google search have proved fruitless. Any ideas what I should be looking for?

(I do have Andale Sans UI replaced with Kochi Mincho in Tools > Options > LibreOffice > Fonts, which I think once fixed this or similar issues but apparently doesn’t any more.)

For starters, what is Asian Phonetic Guide? What does it have to do with LibreOffice?