Hi,
The issue comes from the customization you have on the views of the product page.
The hidden data of the variants is inside the form of the product page instead of being outside, as it is by default.
Because of that, the when you click on the add to cart button, the system on the server side only sees the data of the last variant, regardless of what you choose in the characteristic dropdown.
So you need to check your view overrides, and if you didn't do them, you need to check with the developer who did, or your template provider if the override is provided by your template.