discount problem

[shumy300]

Hi, I created a discount rule with prestashop 1.7 (catalog -> discounts) that does this:
- enter at least 10 of these products in your basket (selected list), apply a 10 € discount.
The rule is correctly applied as soon as I reach 10 pcs between those products but it does a strange thing if I do this:
-a empty cart, I insert any product
-scelgo the list products, 10pz, but the rule does not apply.
It is as if inserting a product "before" into the insertion of the products of that rule would cancel everything.
If you remove the item you choose from the shopping list first, the discount rule is magically applied.

Where am I wrong? is a bug?
Thank you all.