The user needed to show the product variations in the loop template. Everything seemed to be fine until updating the upsells product attached to the variable product that uses the Additional Variation Images extension. After updating when the page loads, especially the first time, the gallery shows the featured image of one of the upsell products.

The problem seems to be that in the js of the Additional Variation Images extension it refers to the DOM element form.variations_form, assuming that there is only one of this form. But if the variations are shown in the loop template, the form is shown too.