I can’t reproduce this. I don’t think it’s just a matter of adding underbrace.
Did you try tweeking mathjax by using the mathjax context menu (right click on the formula?) . If you did it, this may bring mathjax in an error state. We disabled it in 0.8.8.
This may also be a font\resolution problem. Is your screen resolution very low or very high? Did you zoom in or zoom out too much?