Thickbox Close Button Does Not Appear

When using the Staff Directory plugin, and the native popup functionality, the X in the top right shows as a broken image.

When you right click on the image it shows this:

<a href="#" id="TB_closeWindowButton" title="Close"><img src="../wp-includes/js/thickbox/tb-close.png"></a>

Opening that broken image in a new tab goes to:

https://example.com/about-us/staff/wp-includes/js/thickbox/tb-close.png

So clearly, the problem being in the thickbox source, those two periods should be removed so that it always looks for the image at the top directory and not in sub directories. However, it appears that code is in the /wp-includes/js/thickbox.js, in which I removed the two periods and it does work, but that is core code correct? Meaning it could be overrwritten at any time?

Assuming this is a WordPress bug, that would mean the X isn’t showing up for anyone?who uses thickbox on a sub page no?

Thanks in advance.