Hi,
I don't see the characteristic on your link so I can't reproduce the problem and analyze it:
monosnap.com/file/ek8VMeS1usGOBrqDGZ3K4KQXvDqima
However, usually, this is because of the way the product page layout was customized which creates a problem with the HTML when you select a variant. Because of that, you get always the same variant added to the cart.
You can confirm that by renaming the folder templates/YOUR_TEMPLATE/html/com_hikashop/product via FTP. That will disable all the customization of the product page and it should then work properly, confirming that the issue comes from the customization that was done in one of the files of that folder.