Hello,

Make sure you are in mobile view, either in a phone or using the browsers tool. Screen width less than 1024px.

Then when you are visiting the single product which are out of stock, scroll down to the related product grids. There you will find a lot of products with a blue add to cart button. To the left in the button you will see the quantity selector and in the right of the button you will see an eye icon, NOT the basket icon.

If you then visit a product that is in stock and scroll down to the related product section you will see that all products in the grid has a basket icon in the add to cart button, unless sold out products.

So it seems like add to cart button inherit the icon from the single product as when single product is sold out the button icon is an eye.

Brgds
Rune