Why does AJAX API fail to pick certain product IDs when adding to cart?

[david-kaplan]

Hi, community, 

I have a problem with picking selected elements when adding items to the cart using AJAX API. 

When I add products with API I first select all selected items from the form. For some products, the input is selected but the id-data is empty, but the rest of the data are selected. 

It only happens for some products.

 

I the picture you can see that product has Id in the document. 

When I pick that element with JS suddenly the attribute is empty. 

Can you please get me some ideas why is that? 
Thank you 
David