Summary says that all, fontconfig can't pick up the appropriate fonts for Sindhi for Devanagari script due to the lacking of the orth file in it.
How to reproduce:
1. install Samyak Devanagari font.
2. fc-match serif:lang=sd-in@devanagari or :lang=sd@devanagari
DejaVuSans.ttf: "DejaVu Sans" "Book"
Samyak-Devanagari.ttf: "Samyak Devanagari" "Regular"
Aside from no orth file provided in fc-lang/, FcGetDefaultLang() seems ignoring @scriptname in current locale. so if lang= isn't given and run with LANG=sd_IN.UTF-8@devanagari say, fontconfig will set the default lang as sd-in.
*** This bug has been marked as a duplicate of bug 25647 ***