Hi,
I have been doing some excessive testing with my hoster in the past days and after removing all caching, the issue still exists.

See here https://streamable.com

When using the tewntyseventeen theme from wordpress, it works without a problem, so this is probably a theme related issue (s. screenshot attached).

During the investigation, we found out that the problem happens only to variations of products, not when adding a product without variations.

Here’s a product variation you can use (first one, the 1/2017): http://swedenformshop.wpengine.com/produkt/mallikerta/

Here you can find products without variations: http://swedenformshop.wpengine.com/product-category/webliteratur/

Also, when you have a product that is not a variation in the cart and add a product variation of another product, it will replace the product in the AJAX Cart widget but when visiting the cart page, the variation disappeared and only the product that was there before will be in the cart.

There’s also an error showing in the console (s. screenshot #2), it might be related to the issue.