The problem has not yet been solved with version 0.7f of unicode-math
, so this answer is just an attempt to see what's happening; an ugly workaround is proposed.
This is what I get when adding \showlists
to the definition of \big
with
\usepackage{etoolbox}
\makeatletter
\patchcmd{\big}{\n@space}{\n@space\showlists}{}{}
\makeatother
from $\big($
.
unicode-math
version 0.7e
\mathinner
.\left"28000
.\mathord
..\vbox(8.5+0.0)x0.0
.\right"42E000
### restricted horizontal mode entered at line 20
spacefactor 1000
### math mode entered at line 20
### math mode entered at line 20
\mathord
### restricted horizontal mode entered at line 18
spacefactor 1000
### vertical mode entered at line 0
### current page:
\write-{}
unicode-math
version 0.7f
### math mode entered at line 20
\mathinner
.\left"28000
.\mathord
..\vbox(8.5+0.0)x0.0
.\right"0
### restricted horizontal mode entered at line 20
spacefactor 1000
### math mode entered at line 20
### math mode entered at line 20
\mathord
### restricted horizontal mode entered at line 18
spacefactor 1000
### vertical mode entered at line 0
### current page:
\write-{}
The output is obtained with XeLaTeX, but with LuaLaTeX it's similar. We can clearly see
.\right"42E000
with the 0.7e version, that shows exactly a centered dot after \right
. With the 0.7f version we get
.\right"0
that is, the expected null delimiter.
Here's the PDF output from the OP's example.
Output with unicode-math
version 0.7e (and TeX Live 2013)

Output with unicode-math
version 0.7f (and TeX Live 2014)

Where's the magic?
In passing from 0.7e to 0.7f the entry for \period
has been removed from unicode-math-table.tex
, so range=\period
does nothing. If we change range=\period
to range=\mathperiod
(the new name), the centered dot reappears.
Workaround
\documentclass{standalone}
%
\usepackage[math-style=ISO]{unicode-math}
%
\setmainfont{MinionPro-Regular}
%
\setmathfont{LatinModernMath-Regular}
\setmathfont[range=\mathperiod]{MinionPro-Regular}
\setmathfont[range=\mathup/{num,latin,Latin,greek,Greek}]{MinionPro-Regular}
\setmathfont[range=\mathbfup/{num,latin,Latin,greek,Greek}]{MinionPro-Bold}
\setmathfont[range=\mathit/{num,latin,Latin,greek,Greek}]{MinionPro-It}
\setmathfont[range=\mathbfit/{num,latin,Latin,greek,Greek}]{MinionPro-BoldIt}
\Udelcodenum`.=1073741824 % reset the right delcode
\begin{document}
$\bigl(x^2+5\bigr).$
\end{document}

According to my PDF previewer, the final period is in Minion Pro.
range=\period
is issued, when I trace the list built by$\left(x\right.)$
I get\right"42E000
instead of the expected\right"0
that would denote an empty delimiter. This happens also with XeLaTeX.unicode-math
2014/06/30 v0.7f.