Update: it looks like this is a bug dependent on the current cart status.

If the cart is empty, increasing the quantity from the product grid and then clicking “add to cart” will only add 1 item to the cart. If the cart is not empty, increasing the quantity from the product grid and then clicking “add to cart” will add the correct number of items to the grid.

This isn’t related to the product being added. We tested it by trying to add 2 of product A to an empty cart and only 1 was added. Then we went back to the product grid and again increased the quantity to 2 for product A and then clicked “add to cart” and now the cart has a total of 3 of product A: 1 from the first addition and 2 from the second addition.

This must be a bug in the theme. Please help