I found a temporary fix to the problem. By the way, the glitch is caused by pagination bootstrap adding a sort parameter when moving from one page to another. This solution may not be the best, but it works.
This is what I've done:
I copied the original content of /store/storefront/controller/pages/product/category.php (version 1.2.0) to my shopping cart. Then I edited line 321 from this:
$pegination_url = $this->html->getSEOURL('product/category','&path=' . $this->request->get['path'] . '&sort=' . $this->request->get['sort'] . '&page={page}' . '&limit=' . $limit, '&encode');
to this:
$pegination_url = $this->html->getSEOURL('product/category','&path=' . $this->request->get['path'] . '&page={page}' . '&limit=' . $limit, '&encode');