A class is arbitrarily added to the footer element

I am working on a site I started ages ago. I have created a footer element but for some reason the .grid-container class is being added to the footer and keeps the footer contained no matter what. I have regenerated GP and GB cache, there is no other cache on the site. I have also deleted and reinstalled GP + GPP. Nothing suspicious in the Customizer CSS, either, when I remove all CSS, issue persists.

I am attaching a video describing the issue.