1. emacs25 -Q
2. In the *scratch* buffer, M-x fundamental-mode
3. M-: (insert (propertize "ஸ்ரீ ஜம்புநாதன்" 'face
(list :family "Lohit Tamil")))
4. C-u C-x = on the inserted text
5. Note that the font family of the rendered text is *not* 'Lohit Tamil' but a
different, 'Meera Tamil'.  (See the attached screenshot)
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Repeat the above steps, but with emacs24 -Q.  Note that the font family of the
rendered text is the same as the one requested.  (See the attached screenshot)

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I have many different Tamil fonts on my machine, much more than that comes with
stock Debian Sid.  Note that for reproducing the bug, you need to atleast have
a
couple of Tamil fonts installed AND in the emacs-lisp snippet, I shared above,
you need to request a font family that isn't the default one picked by Emacs.

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If you don't mind working with a non-minimal snippet, use
https://github.com/kjambunathan/fontmenu/blob/master/fontmenu.el and follow the
instructions in
https://github.com/kjambunathan/fontmenu/blob/master/README.md.