Why does product size change in the cart for some customers?

[BasicMath]

Hi.

Im experiencing a really weird issue on our site. The problem accure to only a small percentage of our customers, randomly. The problem is, when they try to add a product to the cart, the size changes to the smallest available size. Example: Adding size Large, but size Small ends up in the cart... 

I've talked to Shopify support, Theme developers, Task Husky and almost every app we have and none of the are able to do anything about it. Seems like a big problem is that not everyone is getting/experiencing the problem, then it is hard to fix it... I have talked to some developers that have pointed me in the direction: "the issue lays in the API call that is being sent out which is holding the information going from one place to another". 

www.thebasicshop.no

Something anyone have experienced before and can help me with? Or, send me in the right direction. Starting to get a bit desperate...

Kind regards,
Mathias