Product variants and tax ID and warehouses

-- HikaShop version -- : 3.4.0
-- Joomla version -- : 3.8.7
-- PHP version -- : 7.0

Hi,
my customer has a couply products with a special warehouse and product_tax_id and variants. (Specifically magazine subscriptions with options for national and international shipment, etc.) In the main product, the warehouse and tax_id are correctly set, however when looking in the database, some of the variants have the product_tax_id and product_warehouse_id set, some don't.

When you add one of the products where those are not set to the cart and go to the checkout, it doesn't select the right/any shipment method and buying that cart subsequently fails. I tried saving the variants again and the main product, but that didn't change a thing. I then changed the data in the database directly and that worked.

Is this a bug? How can I prevent this in the future? Where does this come from? I'm a bit confused...

I'd like to make sure that my customer can create other products with variants without having me to go into the DB and fixing values....