When I add “group of 10 quantities” (or any other group) I am still able to add a different amount manually and add it to the cart.
https://www.screencast.com/t/WgOCXLhIP7zs
https://www.screencast.com/t/iUNTLrEKwBrf
I switched theme and then it works as it has to. Contacted WC support allready. They say that Xstore overwrites 35 WC template files. This could cause it. You can try with product http://ebben.jeroenberings.nl/product/nitril-handschoen-zwart. When I use +/- in cart is works like expected (amounts of 10 after each click.
Website is still un development!