Block preorder not working

This is related to the Case #4 – Allow only preorders

When I add a preorder product with variants (all set as preorder), and there are already stock products on the cart, the restriction doesn’t work (allowing mixed products in the order).

The blocking and warning only works properly if you had preorder products on your cart and tries to add in stock products.

I deactivated my theme and all non-essential plugins and still happens.

The page I need help with: [log in to see the link]