mfenced should be equivalent to its expanded form <mrow> <mo fence="true"> opening-fence </mo> <mrow> arg#1 <mo separator="true"> sep#1 </mo> ... <mo separator="true"> sep#(n-1) </mo> arg#n </mrow> <mo fence="true"> closing-fence </mo> </mrow> However, the spacing for mfenced is - .1em before and after the fences - 0em/.25em before/after the separators while the spacing for mo is - .1em before and after the operator when it is the first or last child of an mrow - .2em otherwise. Bug 115787 should solve that issue.
This will be fixed by bug 115787. *** This bug has been marked as a duplicate of bug 115787 ***