Only first product is added to cart

1. Install drupal commerce and inline entity form:
   1. commerce_cart
   2. commerce_checkout
   3. commerce_ui
   4. commerce_line_item_ui
   5. commerce_product_ui
   6. commerce_product_reference
   7. inline_entity_form
2. Add product node
3. Add product reference field to product node.
   1. Set the field as multiple values.
   2. Set inline_entity_form as widget
4. Set inline_entity_form as widget for product line item"Product" field. (/admin/commerce/config/line-items/product/fields/commerce_product/widget-type)
5. Add new product with 3 variations.
6. Go to product page
   
7. Choose a different variation from a default one
   
8. See that the default product was added to the cart
   

Step 4: "Set inline_entity_form as widget for product line item"Product" field." causes the bug.

As not so many users use inline entity form inside line items, the priority of the bug is minor.

I'm not sure if it is a bug of Drupal commerce or Inline entity form.