When “Display Product Seller” seller option is enabled (https://prnt.sc/4rc3eVLJyHlI), we split products in the cart as per each vendor (https://prnt.sc/5aeb-JxXY7VH).

If you don’t want that add this code in the function.php of the theme :

add_filter( 'mvx_display_multiple_vendor_notice_at_cart_chekout_page', '__return_false' );