I just tried it and it seems to be working fine on our demo site. Can you guide us step by step through the steps you are performing to reproduce the problem including what product are you adding to the cart, does this product have required attributes and etc.
There is no quantity to the shopping cart icon on mobile devices by design. Not only that but also a limitation because on mobile the shopping cart icon is a new html element that is added specifically for the theme, and when adding a product to your cart via nopCommerce's ajax cart button, this mobile shopping cart quantity will not update. This is another reason not to place a quantity for mobile devices.
However I think that this is a good suggestion that you can add in our user voice portal.