The first issue is the usage of the staircase kerning boxes of opentype math fonts. They are obviously not applied in expressions like $f_j$ using LuaLaTeX. The index is rendered too far away from the glyph. If we use XeLaTeX we get the correct output.
It works okay with luatex 1.09 (miktex or dev-TeXLive), but fails in a current texlive 2018.
The second issue is present in both LuaLaTeX and XeLaTeX. If we have an index together with an exponent like j the index is rendered too high – in fact it is risen compared to the expression that uses an index only. A typical example is $f_n^j$.
The sup/sub pair is higher placed than with context. You could move it down (at least with luatex) with \documentclass{article} \usepackage{unicode-math} \setmathfont{Cambria Math} \begin{document} $f_n^j$ \Umathsupsubbottommax\textstyle =2pt $f_n^j$ \end{document} unicode-math could probably offer some option to set this. -- Mit freundlichen Grüßen Ulrike Fischer mailto:luatex@nililand.de