Using custom fonts for Tamil

I am trying to use custom unicode fonts for Tamil books, but Moon+ reader seems to revert to the default font (though the text is rendered correctly, but with the default font). How do I find out why Moon+ is having issues using the font I chose?

Custom fonts for English books, seem to work well. Only the fonts that are specifically designed for Tamil language (with basic english character support) seems to have this problem.

Has anyone had success setting a tamil font in Moon+ reader?