When there is an equation with
\tag in it as the one below, then if you left click on the equation, the MathJax border is incorrectly shown.
Here is some random paragraph to show how this text is consumed by MathJax border.
It looks like it is consuming the text preceding the equation as well...
One more example (taken from $x^5 + x^4 +1$ to be a perfect square):
I can see it in Chrome 72.0.3626.121, IE for some reason does not show the border at all. It also seems to happen only with default (HTML-CSS) math renderer.