\documentclass{article}
% Mathias Schickel, 12.12.2018

\usepackage{unicode-math}

\setmainfont{lmroman12-regular.otf}
\setmathfont{latinmodern-math.otf}
		
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\noindent
The index~$n$ in~$f_n^j$ is rendered obviously too high – compare it to~$f_n$ or to~$f_n^m$:
\begin{equation}
	f_n f_n^j f_n^m.
\end{equation}
Why should indices of expressions having exponents with a descender (like~$j$) be rendered higher than indices in expressions having an exponent without a descender (like~$m$)?

In fact the way around would make much more sense because otherwise the descender of the exponent gets very close to the mean line of the index which leads to an uneven and kind of distracting impression and is thus bad typography.

\end{document}