So, something isn't loaded/installed correctly.

Looking at the code sequence of the generated page as compared to the downloaded version of 2.1.2, the GitHub instance of 2.1.2 and the proposed 2.1.3, the includes/modules/pages/product_info/jscript_zen_colorbox.php file has this basic sequence:
load the jquery.colorbox-min.js file
then load the colorbox options, preventing loading of the productMainImageReview image on the main reviews page.
then basically load the link that is to be available.

For some reason on this site, that set of "code" happens after an earlier script load that "attempts" to establish a link to be clicked on as if the display_js_link code was called before the colorbox has been initiated and then also the display_link code is called as well... I don't know how/why the display_js_link code was called, how/why it was to be displayed before the colorbox was initiated and/or directly associated with the object to which action was to be taken.

Basically, the JScript file(s) have been modified away from the default and not in a sequence supporting operation. There may be other JavaScript/jquery issues, but to also add discussion about jquery-1.12.4.min.js, it is set to load *if* jQuery hasn't been loaded, so it should not be in the work space nor have any impact if loaded after the Zen Cart's normal version of jQuery or anything that has already defined jQuery.

In short, I am not seeing that version 2.1.2 is actually installed and don't know what has been modified from that installation to give the layout/configuration currently on the site...