I have a fresh installed Magento 2.1.0. I have created 1 product for testing. The website is running on multiple store, two different URL.
If i clock on "Add to Cart" while inside product details, the shopping cart will be come empty. However if i click on the "Add to Cart" button on product list (thumbnails), then i will see some items in the shopping cart, together with the item added from the product detail.
It's a fresh install, default theme, multiple store with different url, different currency.