For those out there still struggling to get fonts to work on these tags, your ePub is missing an com.apple.ibooks.display-options.xml file in the meta-inf folder.
Add this to the file:
<?xml version="1.0" encoding="UTF-8"?>
<display_options>
<platform name="*">
<option name="specified-fonts">true</option>
</platform>
</display_options>