Spacing in \frac with xelatex and unicode-math

Question

If I compile the following MWE with xelatex, it yields fraction lines at different heights in the second fraction. If I compile it with lualatex, that doesn't happen. If I omit the first fraction, it doesn't happen. If I omit unicode-math, it doesn't happen. (If I omit unicode-math, fontspec and use LaTeX, dvips and ps2pdf, it doesn't happen.)

Is that correct behaviour (if yes, for which engine?) and I should use other spacing code in the fraction (I need blank fractions) or is this a bug (in the other engine)?

\documentclass{article}
\usepackage{fontspec}
\usepackage{unicode-math}
\begin{document}
\(\frac{a}{b}\) \(\frac{\hspace*{1cm}}{\hspace*{1cm}}=\frac{\hspace*{1cm}}{\hspace*{1cm}} \)
\end{document}

looks like a bug to me luatex with fontspec gives the same height bar in all three fractions — David Carlisle, Nov 20 '16 at 10:42
it is unrelated to the blanks, you get the same from \(\frac{a}{b}\) \(\frac{a}{b}\) the first fraction always uses a thicker rule — David Carlisle, Nov 20 '16 at 11:01