I have also tried with the other products, the mini cart does not take the right quantities, therefore it is not incompatible plugins but there is an update problem in the cart, if in any product I try to insert 2 or 3 times various quantities the mini cart keeps adding only 1 unit at a time, can you check please?
1) Go to Appearance > Themes > install and activate the Storefront theme (default official WooCommerce theme). Try to add to cart and check if the Storefront theme mini cart updates correctly. In case you have the same issue with the Storefront then contact plugin author we won’t be able to help you.
2) If everything works fine with the Storefront but the problem persists ONLY once you switch to XStore then provide us with screenshots of the Storefront mini cart content (to explain what products should be there when the plugin works), WP Dashboard access to your site because you use the maintenance mode and we can’t check the site http://prntscr.com/tjh13p and FTP access. We’ll check the compatibility issue.