As it stands right now, in WooCommerce 3.5.1, at product archive (the /shop page), if you use the +/- buttons to set a quantity before clicking "Add to cart", all is fine. But if you try to actually type it in the input#quantity field instead of using the buttons, it adds just one product to cart.
This plugin seems to not have the issue, for single products: https://wordpress.org/plugins/woocommerce-ajax-cart/
This guy has done something similar for the cart page: https://businessbloomer.com/woocommerce-automatically-update-cart-quantity-change/
Using the theme Porto: https://themeforest.net/item/porto-responsive-wordpress-ecommerce-theme/9207399
Please test using this website: https://upvotes.club/product-category/managed-services/
Any idea how to solve it for the product archive as well?