Unicode-math: subscripts work with Bitstream Vera Sans, but not with DejaVu Sans

Question

I'm trying to typeset some slides with text in either Bitstream Vera Sans or DejaVu Sans and maths in Fira Math using LuaLaTeX and unicode-math. When using Bitstream Vera Sans, everything works fine (except for the many glyphs missing in Fira Math):

When using DejaVu Sans, on the other hand, subscripts don't:

What could be the cause of the difference?

\documentclass{minimal}
\usepackage{unicode-math}
\setmathfont{Fira Math}
% \setmathfont[range=up/{latin,Latin}]{Bitstream Vera Sans}
% \setmathfont[range=it/{latin,Latin}]{Bitstream Vera Sans Oblique}
\setmathfont[range=up/{latin,Latin}]{DejaVu Sans}
\setmathfont[range=it/{latin,Latin}]{DejaVu Sans Oblique}
\begin{document}
\[ a_i \]
\end{document}