Please use Help>Check for Updates to install the 15.30 update released this past week, then restart your computer and test again.
If 15.30 doesn't fix it, you can also use Help>Check for Updates to apply to the
Office Insider program. The slow stream should now have the 15.31 update, that's worth installing, I've seen at least one report from a 15.31 user who said the problem was solved.
If the issue persists. please click on the small smiley-face icon in the upper-right corner of the program window, choose
Tell Us What Can Be Better and describe the problem in detail. Submitting sends the bug report to the Mac programming team.