[luatex] Incorrect setting of mu sizes with different styles?

[Joseph Wright]

Hello all,

With the example

    \setbox0=\hbox{$\mkern18mu$}\message{\the\wd0}
    \setbox0=\hbox{$\scriptstyle\mkern18mu$}\message{\the\wd0}
    \bye

pdfTeX gives

    9.99976pt 8.19443pt

whereas LuaTeX gives

    9.99976pt 9.99976pt

which suggests something is not right in terms of the interaction of the
size of the mu unit with math mode style.

Is this a documented change or a bug?
--
Joseph Wright