Sometimes not getting the right number of items

Hey! On this website [ redundant link deleted ] i have enabled 3 options in the store – for 1, 3 and 6 product quantity. Some times when you choose 6 in the cart are not added 6, but only 1. This happens whith the option with 3 quantities. This happens to few people in the team while testing the website. The cache is cleared, the issue happens on multiple atempts and webbrowsers. Can you take a look?

Regards

The page I need help with: [log in to see the link]