The method for regular users is to contact Microsoft Support by phone or email. If they can reproduce the issue they will create the bug report on your behalf.
This is an old question, but one answer today is to use the Feedback feature.
Press the “File” tab near the top left, and select “Feedback from the options on the left hand side.
This will take you to a Feedback application.
I have second-hand experience of consulting Microsoft support. For, other people’s support threads are available on the web and often I find that stuff when searching for a solution to the problem. What I’ve found is that approximately 90% of the time Microsoft support manage in no way whatsoever to solve the problem. Indeed it seems to me that most of the time the support people barely try, or at least do not try with any intelligence.
Office has long standing bugs and, unless one fancies the dialogue with the deaf that the above involves, there is nothing one can do.