Gift card on variable products

Hello,
I’m trying to set gift cards for a variable product. The product page works, but when I add to cart, the to:, to email: and message are not saved, and therefore shown as empty on the cart page, checkout and confirmation email.
could you please check?
It works if I switch this product back to a simple product.

See variable product:
http://www.fitntasty.ch/produit/bon-cadeau-pour-une-cure-detox/

See simple product:
http://www.fitntasty.ch/produit/bon-cadeau-pour-une-cure-detox-2/

https://wordpress.org/plugins/gift-cards-for-woocommerce/