Checkout Express Buttons Not Working correct

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recently I started getting emails from customers that their selected product size differs from the one that the system orders them. This only happens when using Express buttons. Basically what happens is when the customer goes on the product page he selects size. However for some reason when he checkouts with the Express button, the system orders the default size and not the one he has selected.  For some reason this issue happens from time to time - it does not happen on every single order, however, I am losing money from it and need a solution.

I use LuckyOrange to prove that the customer is not responsible for the issue.