I'd like to just report this bug to MS, but I don't see any mechanism to do so.
This happens in both Office 2010 and 2013. I'm using XITS Math as my default math font. (it has much better coverage of characters I use compared with Cambria Math, and more recognizable shapes for characters like \scriptE and \scriptF).
If I start a new equation with alt+= and type 'exp space', it recognizes an exponential function and creates an insertion box, all in XITS Math font. Now if I move the cursor to the right of the insertion box, the font there is set to Cambria Math. This leads to me inadvertently having a jumbled mess of Cambria Math and XITS Math in my equations.